US20080061800A1 - Methods and systems for sigma delta capacitance measuring using shared component - Google Patents
Methods and systems for sigma delta capacitance measuring using shared component Download PDFInfo
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- US20080061800A1 US20080061800A1 US11/925,541 US92554107A US2008061800A1 US 20080061800 A1 US20080061800 A1 US 20080061800A1 US 92554107 A US92554107 A US 92554107A US 2008061800 A1 US2008061800 A1 US 2008061800A1
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03K—PULSE TECHNIQUE
- H03K17/00—Electronic switching or gating, i.e. not by contact-making and –breaking
- H03K17/94—Electronic switching or gating, i.e. not by contact-making and –breaking characterised by the way in which the control signals are generated
- H03K17/945—Proximity switches
- H03K17/955—Proximity switches using a capacitive detector
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R27/00—Arrangements for measuring resistance, reactance, impedance, or electric characteristics derived therefrom
- G01R27/02—Measuring real or complex resistance, reactance, impedance, or other two-pole characteristics derived therefrom, e.g. time constant
- G01R27/26—Measuring inductance or capacitance; Measuring quality factor, e.g. by using the resonance method; Measuring loss factor; Measuring dielectric constants ; Measuring impedance or related variables
- G01R27/2605—Measuring capacitance
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- G—PHYSICS
- G06—COMPUTING; CALCULATING OR COUNTING
- G06F—ELECTRIC DIGITAL DATA PROCESSING
- G06F3/00—Input arrangements for transferring data to be processed into a form capable of being handled by the computer; Output arrangements for transferring data from processing unit to output unit, e.g. interface arrangements
- G06F3/01—Input arrangements or combined input and output arrangements for interaction between user and computer
- G06F3/03—Arrangements for converting the position or the displacement of a member into a coded form
- G06F3/041—Digitisers, e.g. for touch screens or touch pads, characterised by the transducing means
- G06F3/0416—Control or interface arrangements specially adapted for digitisers
- G06F3/04166—Details of scanning methods, e.g. sampling time, grouping of sub areas or time sharing with display driving
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- G—PHYSICS
- G06—COMPUTING; CALCULATING OR COUNTING
- G06F—ELECTRIC DIGITAL DATA PROCESSING
- G06F3/00—Input arrangements for transferring data to be processed into a form capable of being handled by the computer; Output arrangements for transferring data from processing unit to output unit, e.g. interface arrangements
- G06F3/01—Input arrangements or combined input and output arrangements for interaction between user and computer
- G06F3/03—Arrangements for converting the position or the displacement of a member into a coded form
- G06F3/041—Digitisers, e.g. for touch screens or touch pads, characterised by the transducing means
- G06F3/044—Digitisers, e.g. for touch screens or touch pads, characterised by the transducing means by capacitive means
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03K—PULSE TECHNIQUE
- H03K2217/00—Indexing scheme related to electronic switching or gating, i.e. not by contact-making or -breaking covered by H03K17/00
- H03K2217/94—Indexing scheme related to electronic switching or gating, i.e. not by contact-making or -breaking covered by H03K17/00 characterised by the way in which the control signal is generated
- H03K2217/96—Touch switches
- H03K2217/9607—Capacitive touch switches
- H03K2217/96071—Capacitive touch switches characterised by the detection principle
- H03K2217/960725—Charge-transfer
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Abstract
Description
- This application is a continuation-in-part application of U.S. patent application Ser. No. 11/446,324, which was filed on Jun. 3, 2006, which claims priority to U.S. Provisional Patent Application Ser. Nos. 60/687,012; 60/687,148; 60/687,167; 60/687,039; and 60/687,037, which were filed on Jun. 3, 2005 and Ser. No. 60/774,843 which was filed on Feb. 16, 2006, and Ser. No. 60/784,544 which was filed on Mar. 21, 2006, and are incorporated herein by reference.
- This application is also a continuation-in-part application of U.S. patent application Ser. No. 11/446,323, which was filed on Jun. 3, 2006, which claims priority to U.S. Provisional Patent Application Ser. Nos. 60/687,012; 60/687,148; 60/687,167; 60/687,039; and 60/687,037, which were filed on Jun. 3, 2005 and Ser. No. 60/774,843 which was filed on Feb. 16, 2006, and are incorporated herein by reference.
- This application is also a continuation-in-part application of U.S. patent application Ser. No. 11/833,828, which is a continuation application of U.S. patent application Ser. No. 11/445,856, which was filed on Jun. 3, 2006, and issued on Aug. 28, 2007, as U.S. Pat. No. 7,262,609, which claims priority to U.S. Provisional Patent Application Ser. Nos. 60/687,012; 60/687,148; 60/687,167; 60/687,039; and 60/687,037, which were filed on Jun. 3, 2005 and Ser. No. 60/774,843 which was filed on Feb. 16, 2006, and are incorporated herein by reference.
- The present invention generally relates to capacitance sensing, and more particularly relates to devices, systems and methods capable of detecting a measurable capacitances using shared components.
- Proximity sensor devices (also commonly called touch pads or touch sensor devices) are widely used in a variety of electronic systems. A proximity sensor device typically includes a sensing region, often demarked by a surface, to determine the presence, location and/or motion of one or more fingers, styli, and/or other objects. The proximity sensor device, together with finger(s) and/or other object(s), can be used to provide an input to the electronic system. For example, proximity sensor devices are used as input devices for larger computing systems, such as those found integral within notebook computers or peripheral to desktop computers. Proximity sensor devices are also used in smaller systems, including: handheld systems such as personal digital assistants (PDAs), remote controls, communication systems such as wireless telephones and text messaging systems. Increasingly, proximity sensor devices are used in media systems, such as CD, DVD, MP3, video or other media recorders or players.
- Many electronic devices include a user interface, or UI, and an input device for interacting with the UI (e.g., interface navigation). A typical UI includes a display for showing graphical and/or textual elements or other user feedback to input. The display may range from a vector driven CRT, or a scanned LCD, to individually controlled or backlit icons, and a variety of methods of providing visual user feedback. Other forms of feedback to input may also be used (e.g. audio and tactile) to provide modal information, selection acknowledgement, or action timing/ordering information. The increasing use of these types of UIs has led to a rising demand for proximity sensor devices as pointing devices. In these applications the proximity sensor device can function as a value adjustment device, cursor control device, selection device, scrolling device, graphics/character/handwriting input device, menu navigation device, gaming input device, button input device, keyboard and/or other input device.
- Proximity sensor devices and capacitive sensors are commonly used as input devices for computers, personal digital assistants (PDAs), remote controls, media players, video game players, consumer electronics, cellular phones, payphones, point-of-sale terminals, automatic teller machines, kiosks and the like. Many proximity sensors use measurements of capacitance to detect object position or proximity (or motion or presence or any similar positional information). Capacitive sensing techniques are used in user input buttons, slide controls, scroll rings, scroll strips and other types of sensors. One other type of capacitance sensor used in such applications is the button-type sensor, which can be used to provide information about the existence or presence of an input. As discussed above, another type of capacitance sensor used in such applications is the touchpad-type sensor, which can be used to provide information about an input such as the position, motion, and/or similar information along one axis (1-D sensor), two axes (2-D sensor), or more axes. Both the button-type and touchpad-type sensors can also optionally be configured to provide additional information such as some indication of the force, duration, finger count, finger separation, or amount of capacitive coupling associated with the input. Imaging sensors with independent arrays of sensors may be used for multiple inputs for multi-finger position sensing, or for measuring relative motions. A variety of reporting, signaling, and methods of data reduction before transmission, as well as, polled or interrupt control methods may be used. A variety of fabrication and assembly methods may also be used including, flexible and rigid printed circuit boards, transparent conductors and substrates, as well as, methods of creating interconnects and vias. One example of a 2-D touchpad-type sensor that is based on capacitive sensing technologies is described in U.S. Pat. No. 5,880,411, which issued to Gillespie et al. on Mar. 9, 1999. Such sensors can be readily found, for example, in input devices of electronic systems including handheld and notebook-type computers.
- A user generally operates a capacitive input device by placing or moving one or more fingers, styli, and/or objects, near a sensing region of one or more sensors located on or in the input device. This creates a capacitive effect upon a carrier signal applied to the sensing region that can be detected and correlated to positional information (such as the position(s) or proximity or motion or presences or similar information) of the stimulus/stimuli with respect to the sensing region. This positional information can in turn be used to select, move, scroll, or manipulate any combination of text, graphics, cursors and highlighters, and/or any other indicator on a display screen. Other visual, auditory, and tactile feedback methods can also be used for highlighting or display. The positional and/or temporal information can also be used to enable the user to interact with an interface, such as to control volume, to adjust brightness, or to achieve any other purpose.
- Although capacitance sensors have been widely adopted for several years, sensor designers continue to look for ways to improve the sensors' functionality and effectiveness. In particular, engineers continually strive to improve the performance for the design and implementation of position sensors without increasing costs, pin count, electrode routing, component count, size, or complexity. Moreover, as such sensors become increasingly in demand in various types of electronic devices, a need for a highly-flexible yet low cost and easy to implement sensor design arises. In particular, a need exists for a sensor design scheme that is flexible enough for a variety of implementations and powerful enough to provide accurate capacitance sensing while remaining cost effective.
- Accordingly, it is desirable to provide systems and methods for quickly, effectively and efficiently detecting a measurable capacitance. Moreover, it is desirable to create a design scheme that can be readily implemented using readily available components, such as standard ICs, microcontrollers, and discrete components. Other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.
- Methods, systems and devices are described for detecting a measurable capacitance using sigma-delta charge transfer techniques that can be implemented with many standard microcontrollers, and can share components to reduce device complexity and improve performance. In these embodiments, a voltage is repeatedly applied to each measurable capacitance, and the measurable capacitance is allowed to share charge with a passive network such that the passive network accumulates charge on at least one integrating capacitance. If the charge on the passive network is past a threshold value, then the charge on the passive network is changed by a quantized amount and the process is repeated. The results of the charge threshold detection are a quantized measurement of the accumulated shared charge, which can be filtered to yield a measure of the measurable capacitance. In the various implementations of this embodiment, the passive network used to accumulate charge can be shared between multiple measurable capacitances. A switch or IO controlling (or other elements comprising) the charge sharing and/or charge changing circuit can also be shared. Likewise, in various implementations a voltage conditioning circuit configured to provide a variable reference voltage can be shared between multiple measurable capacitances. Finally, in various implementations a guarding electrode configured to guard the measurable capacitances can be shared between multiple measurable capacitances. In each of these cases, sharing components can reduce device complexity and improve performance.
- Such a detection scheme may be readily implemented using readily available components, and can be particularly useful in sensing the position of a finger, stylus or other object with respect to a capacitive sensor implementing button function(s), slider function(s), cursor control or user interface navigation function(s), or any other functions.
- Various aspects of the present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and
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FIG. 1 is a schematic diagram of an exemplary proximity sensor device with an electronic system; -
FIG. 2 is a schematic diagram of an exemplary proximity sensor device showing component sharing; -
FIGS. 3A-3C are schematic views of capacitive sensors using charge transfer in accordance with an embodiment of the invention sharing an integrating capacitance; - FIGS. 4A-B are schematic views of a capacitive sensors using charge transfer in accordance with an embodiment of the invention a sharing voltage conditioning circuit;
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FIG. 5 is a schematic view of a capacitive sensor using charge transfer in accordance with an embodiment of the invention sharing a guarding electrode; -
FIGS. 6 and 7 are graphical views of charge transfer timing schemes in accordance with an embodiment of the invention; -
FIG. 8 is a schematic view of a capacitive sensor using sigma delta capacitive detection in accordance with an embodiment of the invention sharing a delta capacitance and an integrating capacitance; -
FIGS. 9A-9C are schematic views of capacitive sensors using sigma delta capacitive detection in accordance with an embodiment of the invention sharing voltage conditioning; -
FIGS. 100A-10C are schematic views of capacitive sensors using sigma delta capacitive detection in accordance with an embodiment of the invention sharing a guarding electrode; -
FIG. 11 is a state diagram illustrating operation of a capacitive sensor in accordance with an embodiment of the invention; -
FIG. 12 is a timing diagram illustrating operation of a sigma delta capacitive sensor in accordance with an embodiment of the invention; -
FIG. 13 a schematic view of a capacitive sensor using charge transfer in accordance with an embodiment of the invention sharing an integrating capacitance; -
FIG. 14 is a schematic view of a capacitive sensor using charge transfer in accordance with an embodiment of the invention sharing integrating capacitances; -
FIG. 15 is a schematic view of a capacitive sensor using sigma delta capacitive detection and sharing multiplexer in accordance with an embodiment of the invention; -
FIG. 16 is a schematic view of a capacitive sensor using sigma delta capacitive detection and sharing a multiplexer and a dithering circuit in accordance with an embodiment of the invention; -
FIG. 17 is a schematic view of a capacitive sensor using sigma delta capacitive detection in accordance with an embodiment of the invention sharing an IO for charge changing; - FIGS. 18A-B are schematic views of a capacitive sensor using sigma delta capacitive detection for transcapacitive measurement in accordance with an embodiment of the invention.
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FIG. 18C is a timing diagram of a capacitive sensor using sigma delta capacitive detection for transcapacitive measurement in accordance with an embodiment of the invention. - The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description.
- Methods, systems and devices are described for detecting a measurable capacitance using charge transfer techniques that can be implemented with many standard microcontrollers, and can share components to reduce device complexity and improve performance. The methods, systems and devices for detecting measurable capacitances can be utilized in a variety of different applications, including in proximity sensor devices.
- Turning now to the drawing figures,
FIG. 1 is a block diagram of an exemplaryelectronic system 100 that is coupled to aproximity sensor device 116.Electronic system 100 is meant to represent any type of personal computer, portable computer, workstation, personal digital assistant, video game player, communication device (including wireless phones and messaging devices), media device, including recorders and players (including televisions, cable boxes, music players, and video players) or other device capable of accepting input from a user and of processing information. Accordingly, the various embodiments ofsystem 100 may include any type of processor, memory or display. Additionally, the elements ofsystem 100 may communicate via a bus, network or other wired or wireless interconnection. Theproximity sensor device 116 can be connected to thesystem 100 through any type of interface or connection, including I2C, SPI, PS/2, Universal Serial Bus (USB), Bluetooth, RF, IRDA, or any other type of wired or wireless connection to list several non-limiting examples. -
Proximity sensor device 116 includes acontroller 119 and asensing region 118.Proximity sensor device 116 is sensitive to the position of astylus 114, finger and/or other input object within thesensing region 118. “Sensing region” 118 as used herein is intended to broadly encompass any space above, around, in and/or near theproximity sensor device 116 wherein the sensor of the touchpad is able to detect a position of the object. In a conventional embodiment,sensing region 118 extends from the surface of the sensor in one or more directions for a distance into space until signal-to-noise ratios prevent object detection. This distance may be on the order of less than a millimeter, millimeters, centimeters, or more, and may vary significantly with the type of position sensing technology used and the accuracy desired. Spatial and temporal filters may be adapted (e.g. different electrode patterns, or changed threshold) for different distances corresponding to proximity, and touch. Different results (e.g. presence, motion, or location) and effects (e.g. wake up from low power, or cursor motion) can result from changing user input, or from different sensor designs. Accordingly, the planarity, size, shape and exact locations of theparticular sensing regions 116 will vary widely from embodiment to embodiment. In some designs portions of the sensing region may be configured to provide visual, aural, or tactile feedback to a user (e.g. through markings, texture, or shape), and may limit the motion of the input, or provide alignment features for it. - In operation,
proximity sensor device 116 suitably detects a position ofstylus 114, finger or other input object withinsensing region 118, and usingcontroller 119, provides electrical or electronic indicia of the position to theelectronic system 100. Thesystem 100 appropriately processes the indicia to accept inputs from the user, to move a cursor or other object on a display, or for any other purpose. - The
proximity sensor device 116 uses measurements of capacitance to detect the presence of an object or objects. For example, by detecting changes in capacitance caused by the changes in the electric field due to the object. Theproximity sensor device 116 detects the presence of the object and delivers positional information to thesystem 100. The positional information provided by theproximity sensor device 116 can be any suitable indicia of object presence. For example, theproximity sensor device 116 can be implemented to provide “zero-dimensional” 1-bit positional information, “one-dimensional” positional information (e.g. along a sensing region) as a scalar, “two-dimensional” or “three-dimensional” vector positional information (e.g. horizontal/vertical/depth axes, angular/radial axes, or any other combination of axes that span the two or three dimensions) as a combination of values, and the like. Furthermore, the term “positional information” as used herein is intended to broadly encompass absolute and relative position-type information, and also other types of spatial-domain information such as velocity, acceleration, and the like, including measurement of motion in one or more directions of one or more independent input objects. Various forms of positional information may also include time history components and/or a count or relative location of input objects, as in the case of gesture recognition and the like. - The
controller 119, sometimes referred to as a proximity sensor processor or touch sensor controller, is coupled to the sensor and theelectronic system 100. In general, thecontroller 119 receives electrical signals from the sensor (although it may also apply them), processes the electrical signals, and communicates with the electronic system. Thecontroller 119 can perform a variety of processes on the signals received from the sensor to implement theproximity sensor device 116. For example, thecontroller 119 can select or connect individual sensor electrodes, detect presence/proximity, calculate and filter to estimate position or motion information, and report positional information when a threshold is reached, and/or interpret and wait for a valid touch/tap/stroke/character/button/gesture sequence before reporting it to theelectronic system 100, or indicating it to the user. - In this specification, the term “controller” is defined to include one or more processing elements that are adapted to perform the recited operations. Thus, the
controller 119 can comprise all or part of one or more integrated circuits, firmware code, and/or software code that receive or apply electrical signals from or to the sensor and communicate with theelectronic system 100. In some embodiments, the elements that comprise thecontroller 119 would be located with or near the sensor. In other embodiments, some elements of thecontroller 119 would be with the sensor and other elements of thecontroller 119 would reside on or near theelectronic system 100. In this embodiment minimal processing could be performed near the sensor, with the majority of the processing performed on theelectronic system 100. - Furthermore, the
controller 119 can be physically separate from the part of the electronic system that it communicates with, or thecontroller 119 can be implemented integrally with that part of the electronic system. For example, thecontroller 119 can reside at least partially on a processor performing other functions for the electronic system aside from implementing theproximity sensor device 116. - Again, as the term is used in this application, the term “electronic system” broadly refers to any type of device that communicates with
proximity sensor device 116. Theelectronic system 100 could thus comprise any type of device or devices in which a touch sensor device can be implemented in or coupled to. The proximity sensor device could be implemented as part of theelectronic system 100, or coupled to the electronic system using any suitable technique. As non-limiting examples theelectronic system 100 could thus comprise any type of computing device, media player, communication device, or another input device (such as another touch sensor device or keypad). In some cases theelectronic system 100 is itself a peripheral to a larger system. For example, theelectronic system 100 could be a data input or output device, such as a remote control or display device, that communicates with a computer or media system (e.g., remote control for television) using a suitable wired or wireless technique. It should also be noted that the various elements (processor, memory, etc.) of theelectronic system 100 could be implemented as part of an overall system, as part of the touch sensor device, or as a combination thereof. Additionally, theelectronic system 100 could be a host or a slave to theproximity sensor device 116. - In the illustrated embodiment the
proximity sensor device 116 is implemented withbuttons 120. Thebuttons 120 can be implemented to provide additional input functionality to theproximity sensor device 116. For example, thebuttons 120 can be used to facilitate selection of items using theproximity sensor device 116. Of course, this is just one example of how additional input functionality can be added to theproximity sensor device 116, and in other implementations theproximity sensor device 116 could include alternate or additional input devices, such as physical or virtual switches, or additional proximity sensing regions. The addition inputs 120 (e.g. tactile switches) may also be mounted below the surface of theinput device 116 such their activation may occur while theinput object 114 contacts a dielectric or approaches an electrode defining thesensing area 118. Conversely, theproximity sensor device 116 can be implemented with no additional input devices. - It should be noted that although the various embodiments described herein are referred to as proximity sensor devices, these terms as used herein are intended to encompass not only conventional proximity sensor devices, but also a broad range of equivalent devices that are capable of detecting the position of a one or more fingers, pointers, styli and/or other objects. The response to proximity or touch may be different, and the response may also be modal or user configured (e.g. wake on proximity, indicate function on hover, operate on contact). Such devices may include, without limitation, touch screens, touch pads, touch tablets, biometric authentication devices, handwriting or character recognition devices, and the like.
- The various embodiments of the invention provide methods and devices for measuring measurable capacitances that can be utilized in proximity sensor devices. The methods and devices utilize charge transfer techniques that can be implemented with many standard microcontrollers, and can share components to reduce device complexity and improve performance. Turning now to
FIG. 2 ,FIG. 2 is a block diagram of an exemplarycapacitive measuring system 200 that can be utilized in a proximity sensor device. Thecapacitive measuring system 200 includes acontroller 202, sharedcomponents 204 and a plurality ofmeasurable capacitances 206. According to a first embodiment, thecontroller 202 is adapted to perform a charge transfer process to measure each of the plurality of capacitances. The charge transfer process comprises repeatedly applying a pre-determined voltage to the measurable capacitance, and then allowing the measurable capacitance to share charge with a passive network that includes at least one integrating capacitance statically coupled to a plurality of measurable capacitances and configured to accumulate charge received from the plurality of measurable capacitances over repeated cycles. The value of the measurable capacitance can then be determined as a function of a representation of a charge on the filter capacitance and the number of times that the charge transfer process was performed. - According to a second embodiment, the
controller 202 is adapted to repeatedly apply a voltage to each measurable capacitance in the plurality ofmeasurable capacitances 206, and the measurable capacitance is allowed to share charge with a passive network such that the passive network accumulates charge on at least one integrating capacitance over repeated cycles. If the charge on the passive network is past a threshold value, then the charge on the passive network is changed by a quantized amount and the process is repeated. The results of the charge threshold detection are a quantized measurement of the accumulated charge, which can be filtered to yield a measure of the measurable capacitance. It should be noted that the plurality of measurable capacitances can include both absolute capacitances (e.g., capacitance measured from a sensing node to a local ground) and transcapacitances (e.g., capacitance measured between a driving node and a sensing node). - In the various implementations of these two embodiments, a variety of components in the
capacitive measuring system 200 can be shared among the plurality ofmeasurable capacitances 206. For example, the sharedcomponents 204 can include the passive network used to accumulate charge. Likewise, in various implementations the sharedcomponents 204 can include a voltage conditioning circuit configured to provide a variable reference voltage. Additionally, in various implementations the sharedcomponents 204 can include a guarding electrode configured to guard the measurable capacitances. In each of these cases, sharing of sharedcomponents 204 can reduce device complexity (e.g. size or component count) and improve performance (e.g. matching). - Turning now to
FIG. 3A , a first embodiment of acapacitive sensor 300 is illustrated. Thecapacitive sensor 300 is adapted to perform a charge transfer process to measure each of a plurality of measurable capacitances 312. The charge transfer process comprises applying a pre-determined voltage to the measurable capacitance 312, and then allowing the measurable capacitance to share charge with a sharedpassive network 311 that includes at least one integratingcapacitance 310, with the integratingcapacitance 310 statically coupled to a plurality of measurable capacitances 312 and configured to accumulate charge received from the plurality of measurable capacitances 312. The value of eachmeasurable capacitance 312A-D can then be determined as a function of a representation of a charge on the integratingcapacitance 310 and the number of times that the charge transfer process was performed. The number of times that the charge transfer process is executed can be pre-established or be based on the representation of the charge reaching some threshold. The representation of the charge on the integrating capacitance can be obtained by a measuring step that quantizes a representation of charge on the integrating capacitance. These steps can be repeated, and the results of the measuring step can be stored and/or filtered as appropriate, and used for object detection in a proximity sensor device. - In the illustrated embodiment, the integrating
capacitance 310 is shared between four sensing channels, where each of the four sensing channels corresponds to one of themeasurable capacitances 312A-D. Each I/O 304A-D is coupled to ameasurable capacitance 312A-D and to a correspondingpassive impedance 305D. A sharedpassive network 311 is coupled to each of thepassive impedances 305D, with the sharedpassive network 311 including a shared integratingcapacitance 310. It should be noted that the integratingcapacitance 310 is statically coupled to the measurable capacitances 312, without a switch or other active element between them. Instead, the passive impedances 305 serve to couple measurable capacitances 312 to the integratingcapacitance 310, and also serve to selectively (share) transfer charge from the measured capacitances to the integrating capacitor over longer timescales while reducing the leakage of charge due to the application of the charging voltage to the measured capacitances over a shorter time scale. The integratingcapacitance 310 is typically a relatively large capacitance. By using a shared integratingcapacitance 310, statically coupled to multiple sensing channels, thecapacitive sensor 300 facilitates a reduction in device complexity and cost. - It should also be noted that the
capacitive sensor 300 requires only a single I/O 304A-D oncontroller 302 for eachmeasurable capacitance 312A-D. By using only one I/O 304 for each measurable capacitance, this embodiment allows for an even more efficient implementation of a capacitance sensor in terms of I/O usage, and thus is especially useful for large, multi-sensing-channel implementations. For example, a proximity sensor with 20 sensing electrodes could be implemented using 20 I/O's on a controller. In this illustrated embodiment, a single I/O 304 is used to apply the pre-determined voltage to its respective measurable capacitance 312 (e.g. I/O 304A is associated withmeasurable capacitance 312A, and I/O 304B is associated withmeasurable capacitance 312B, etc.), to read the voltage on the shared integratingcapacitance 310, and also to reset the charge on the shared integratingcapacitance 310. - Thus in this one embodiment, the
capacitive sensor 300 measures the plurality ofmeasurable capacitances 312A-D sequentially and one at a time. During the sequential operation, thecontroller 302 applies a charging pulse of a predetermined voltage to a measurable capacitance 312 through the corresponding I/O 304 while other voltage applying I/Os are left in a high impedance state. Between charging pulses, the I/Os 304A-D are set to a high impedance state such that the charged measurable capacitance 312 is allowed to share charge with the shared integratingcapacitance 310 through its corresponding passive impedance 305. This process is repeated, such that charge from multiple charging processes is accumulated on the shared integrating capacitance. - A representation of the accumulated charge (e.g. a voltage) on the integrating
capacitance 310 is then measured using one of the I/Os 304A-D in this simple embodiment. This representation provides a measurement of the measurable capacitance 312 that can be used in an object proximity sensor. It should also be noted that in this specific embodiment, the voltage at the integratingcapacitance 310 cannot be measured directly, as that node is not coupled directly to an I/O 304 of thecontroller 302. However, the voltage at an I/O 304 can be used as the representation of the accumulated charge on integratingcapacitance 310 and thus can be used to determine the capacitance of the measurable capacitance 312. - For example, in one embodiment the voltage at on or more I/Os 304 can be compared to a threshold voltage VTH. The accumulated charge on the integrating capacitance may be affected by the current leakage through the passive impedance during the charging pulse, and this may be compensated for by applying opposing charging pulses (which are not shared) or by estimating the effect and accounting for it by calculation. Further, in some embodiments where the charging voltage is applied by the I/O 304 for a short time (relative to the time constant of the passive impedance and the integrating capacitor) and at certain times (e.g. after the shared charge has almost completely accumulated through the passive impedance) the voltage at the I/O 304 is essentially equivalent to the voltage on the integrating
capacitance 310, such that no difference in measurements exist and needs to be accounted for. - As another example alternative, measurable capacitance 312 may be charged or discharged for a pre-set number of performances of the charge transfer process, with the voltage on integrating
capacitance 310 being measured as a quantized multi-bit value after the pre-set number of performances rather than as a single-bit value by comparison with a threshold. - At the end of a charging cycle (e.g. after the voltage at an I/O 304 exceeds the threshold voltage VTH), one or more of the I/Os 304 applies a suitable reset voltage to reset the charge on the integrating
capacitance 310. For example, an I/O 304 will repeat the process of initially applying pulses of logic “high” values to provide a positive charge to measurable capacitance that is then shared with integratingcapacitance 310, and then at the end of those processes the same I/O resets the integratingcapacitance 310 by driving a logic “low” or “ground” for a period of time sufficient to discharge integratingcapacitance 310 through the passive impedance 305 to a known reset voltage. It should be noted that when the integratingcapacitance 310 is reset through a single passive impedance 305, a relatively long reset period must be used. When many applying I/Os 304 are available it then usually makes sense for them to either be shared during the resetting process, or to use an additional shared reset I/O such as 316 inFIG. 3C to reset the shared integrating capacitance for more than one of the measured capacitances 312. - It should be noted that while the shared integrating
capacitance 310 is illustrated inFIG. 3A as comprising one capacitor, in other embodiments it can be implemented with multiple capacitors that are shared between the various channels. Furthermore, it should be noted that while the illustrated embodiment shows separate passive impedances 305 for each channel, that in some embodiments the sharedpassive network 311 will also include impedances that are shared between channels. - Turning now to
FIG. 3B , a variation on the embodiment ofFIG. 3 is illustrated where acapacitive sensor 350 utilizes an integrating capacitance that comprises twocapacitances capacitances FIG. 4 . - Note also that a variety of alternate embodiments are possible. In addition to the combination of integrating
capacitances 310 illustrated inFIG. 3B for voltage conditioning, it is also possible to reduce the time constant for resetting the integrating capacitance by using multiple I/Os. For example, by using more than one of the voltage applying I/Os to reset the integrating capacitance each through their respective passive impedance as described above or by using one or more I/Os directly connected to the integrating capacitance (e.g. 316, or 316 and 318). Turning directly toFIG. 3C , acapacitive sensor 375 is illustrated that facilitates such a reset.Capacitive sensor 375 also includes a sharedreference channel 313 that includes an impedance ZX4 and acapacitor 314 coupled to the I/O 319. Additionally, thecapacitive sensor 375 includes shared I/O 318 for low side comparison of the integrating capacitor voltage. The reference channel may be used to compensate for variations in operating conditions (e.g. power supply voltage, threshold voltage, printed circuit board dielectric, etc.). In an alternate embodiment thereference channel capacitance 314 may comprise an element of a charge changing circuit wherereference channel 313 can change a charge on an integrating capacitance by a quantized amount of charge. Such a charge changing circuit might be used for offsetting the measurement of a capacitance to increase dynamic range, or for a sigma-delta capacitance measurement technique. - The reset may be provided by a shared reset I/
O 316. The shared reset I/O 316 is connected between the isolatingpassive impedances 305A-C and the integratingcapacitance 310 to provide a rapid reset for more than one of the sensing channels. To perform a reset in one embodiment, the shared reset I/O 316 is driven to a predetermined reset voltage (e.g. ground) while the other side of the integrating capacitance is also driven to a predetermined voltage (e.g. I/O 318 could be omitted and/or that node directly coupled to ground), setting the charge from the integratingcapacitance 310 to a known value (e.g. zero). The reset I/O 316 connected between the isolating impedances and the integrating capacitance could also be used to measure the voltage (and charge) on the integrating capacitance. In both cases the reset I/O 316 is shared between the multiple measurable capacitances. - In another embodiment the shared I/
O 318 coupled to the opposite node of the integratingcapacitance 310 from I/O 316 facilitates a charge measurement technique that provides improved resistance to noise. Specifically, during charge transfer the I/O 318 is driven to Vdd, and thus functions in a similar manner tocapacitive sensor 300 inFIG. 3A , but referenced to a different power supply voltage during charge transfer. Then, when the charge on the integratingcapacitance 310 is to be measured, two of the I/Os (e.g., I/O 304A and I/O 304B) can be driven to VDD while the other two (e.g., I/O 304C and I/O 304D) are driven to ground. This creates a voltage divider that puts a conditioned voltage of ½ VDD at the top of integratingcapacitance 310. The voltage at shared I/O 318 can then be measured relative to the conditioned voltage, providing an accurate measurement of the charge on the integratingcapacitance 310 relative to a threshold voltage that varies with ½ VDD. Additionally, driving the I/Os during measurement this way prevents noise from themeasurable capacitances 312A-C from propagating to the integratingcapacitance 310, thus reducing the effects of that noise on the measurement. Note that in this other embodiment I/O 316 need not be used (or can remain a high impedance input). - In yet another embodiment both I/
Os capacitance 310. The I/O 318 may be grounded during the charge transfer process, while 318 floats at high impedance. Then the I/O 316 may be driven to a stable voltage (e.g. Vdd) while the charge on the integrating capacitance is quantized (e.g. by measuring the voltage on I/O 318 relative to a CMOS threshold) to facilitate determining one or more of themeasurable capacitances 305A-C. - The embodiments illustrated in
FIGS. 3A, 3B , and 3C are some examples of how shared components can be used in a capacitance sensor. Turning now toFIG. 4A , a secondembodiment capacitive sensor 400 is illustrated. In this embodiment thecapacitive sensor 400 shares avoltage conditioning circuit 425 that is adapted to provide a variable reference voltage. For example, a variable reference voltage that varies with a threshold voltage of thecontroller 402. This embodiment does not, however, share an integrating capacitance, and instead uses a plurality of integrating capacitances 410A&B with a sharedvoltage conditioning circuit 425 allowing multiple measurable capacitances (e.g. 412A&B) to be measured concurrently. This is distinct from thesensor 350 inFIG. 3B wheremultiple capacitors capacitance 310 which also provides voltage conditioning without additional circuitry. - Like the embodiment illustrated in FIGS. 3A-C, the
capacitive sensor 400 is adapted to perform a charge transfer process to measure each of a plurality of measurable capacitances 412. The charge transfer process comprises applying a pre-determined voltage to the measurable capacitance 412, and then allowing the measurable capacitance to share charge with a passive network that includes at least one integrating capacitance 410, with the integrating capacitance 410 statically coupled to a plurality of measurable capacitances 412 and configured to accumulate charge received from the plurality of measurable capacitances 412. The value of eachmeasurable capacitance 412A-B can then be determined as a function of a representation of a charge on the corresponding integrating capacitance 410 and the number of times that the charge transfer process was performed. The number of times that the charge transfer process is executed can be pre-established or be based on the representation of the charge reaching some threshold. The representation of the charge on the integrating capacitance can be obtained by a measuring step that quantizes a representation of charge on the integrating capacitance. These steps can be repeated, and the results of the measuring step can be stored and/or filtered as appropriate, and used for object detection in a proximity sensor device. - In the illustrated embodiment of
FIG. 4A , avoltage conditioning circuit 425 is shared between the two measurable capacitances 402A-B. Specifically, thevoltage conditioning circuit 425 includes afirst impedance 427 and asecond impedance 429 coupled as a voltage divider between VDD and ground. As such, thevoltage conditioning circuit 425 provides a variable reference voltage tonode 407. Specifically, thevoltage conditioning circuit 425 provides a voltage that varies with a threshold voltage of thecontroller 402. - The
voltage conditioning circuit 425 is coupled to the side of integrating capacitances 410 opposite the measurable capacitances 412, and to either or both power supply rails (coupling to +VDD and ground) associated with the implementation ofcapacitance sensor 400. With the configuration shown inFIG. 4 , fluctuations in the supply rails (also “power supply voltage ripple”) induce similar fluctuations in the voltage atnode 407, and therefore can be used to compensate for fluctuations in measurement thresholds associated withcontroller 402 induced by the same supply voltage ripple. Specifically, providing a conditioned voltage to the integrating capacitor that varies ratiometrically with the power supply (and with the threshold voltage) compensates for fluctuations in the controller thresholds which have the same ratiometric response. That is, if the voltage reference of the integrating capacitance moves by a similar voltage and at a similar time with the thresholds used to measure the voltage on the integrating capacitance then the variation in the power supply voltage can be compensated for and its effect on the measurement of the capacitance reduced. - The
exemplary compensation circuit 425 includes twoimpedances - For
compensation circuit 425 shown inFIG. 4A , the impedance divider can be a voltage divider formed from two resistances or two capacitances coupled to +VDD and ground. The impedance divider ofcircuit 425 has a “common node” coupled to the integratingcapacitances 410A-B atnode 407. Resistive versions ofimpedances impedances impedances voltage conditioning circuit 425. This is because such a voltage divider provides a voltage that reflects the fluctuations in power supply voltage without significant lag similar to that of the I/O thresholds. - In another example, a conditioned voltage can be provided by a resistance in series with a capacitance (low impedance compared to the total coupled integrating capacitance and the series passive impedances) and an I/O that charges the capacitance and measures its voltage through a resistor to control it to the voltage threshold of the reference I/O. In this case, when the threshold of the reference I/O tracks that of the other measurement I/Os, the conditioned voltage will track the other measurement I/Os. This allows the voltage on the integrating capacitance to be changed so that other measurement I/Os don't spend much time near threshold where they can consume significantly more power.
- Turning now to
FIG. 4B , another embodiment of acapacitive sensor 475 is illustrated. Like thesensor 400, thecapacitive sensor 450 includes avoltage conditioning circuit 475 that shared between the two measurable capacitances 402A-B. Thevoltage conditioning circuit 475 includes afirst impedance 427 and asecond impedance 429 coupled as a voltage divider between VDD and ground. The voltage conditioning circuit also includes I/O 480 coupled tonode 407 through an I/O impedance 479. Thevoltage conditioning circuit 475 provides a variable reference voltage tonode 407. Specifically, thevoltage conditioning circuit 475 provides a voltage that varies with a threshold voltage of thecontroller 402 - Additionally, the I/
O 480 and I/O impedance 479 allows the compensated voltage that is applied tonode 407 to be dynamically changed responsive to a change in the output of I/O 480. For example, if the afirst impedance 427, thesecond impedance 429, and the I/O impedance 479 all have the same value, then the compensated voltage provided tonode 407 will be approximately ⅔ VDD when I/O 480 is driven high, and will be approximately ⅓ VDD when I/O 480 is driven low. This can be useful in applications where I/O thresholds may change, such as in devices that use Schmidt triggers that have different upper and lower thresholds. - The example voltage conditioning circuits illustrated in
FIGS. 4A and 4B are two examples of the type of voltage conditioning circuit that can be shared between measurable capacitances in a capacitive sensor. It should also be noted that while the voltage conditioning circuit is illustrated as being shared between twomeasurable capacitances 412A-B, that this concept can be expanded to share the voltage conditioning circuit between more measurable capacitances to further enhance device component usage efficiency. - Again, the embodiment illustrated in
FIG. 4A -B is just one example of how shared components can be used in a capacitance sensor. Turning now toFIG. 5 , a thirdembodiment capacitive sensor 500 is illustrated. In this embodiment thecapacitive sensor 500 shares a guardingelectrode 503 that is provided a guard signal during at least a portion of the time in which voltage is applied to sensing electrodes 501 and during at least a portion of the time in which the electrodes share charge with an integrating capacitance in a passive network. This embodiment does not, however, share an integrating capacitance, and instead uses a plurality of integrating capacitances 510. - Like the embodiment illustrated in
FIGS. 3-4 , thecapacitive sensor 500 is adapted to perform a charge transfer process to measure each of a plurality of measurable capacitances, where the measurable capacitances are defined, at least in part, by a plurality ofsensor electrodes 501A-B and an object, such as a finger or stylus (not shown), proximate thesensor electrodes 501A-B. The charge transfer process comprises applying a pre-determined voltage to the electrodes 501 via I/Os 504, and then allowing the electrodes 501 to share charge with a passive network that includes at least one integratingcapacitance 510A-B corresponding to the sensor electrodes, with each integrating capacitance 510 statically coupled to an associated electrode 501 through node 513 andimpedance 505, where the integrating capacitances are configured to accumulate charge received from the plurality of measurable capacitances 501. The value of a measurable capacitance at each electrode 501 can then be determined as a function of a representation of a charge on the corresponding integrating capacitance 510 and the number of times that the charge transfer process was performed. The number of times that the charge transfer process is executed can be pre-established or instead can be based on the representation of the charge reaching some threshold. The representation of the charge on the integrating capacitance can be obtained by a measuring step that quantizes a representation of charge on the integrating capacitance. These steps can be repeated, and the results of the measuring step can be stored and/or filtered as appropriate, and used for object detection in a proximity sensor device. - This embodiment also includes a shared
guarding electrode 503 that serves to shield the sensor electrodes 501 from unintended electrical coupling. For example, unwanted electrical coupling can occur between pairs of sensors, between the sensors and external sources (e.g. grounded, stationary relative to ground, or varying with respect to ground), and between sensors and internal sources (e.g. I/Os for LED control or communications). Such unwanted coupling can cause both offsets and variations in the measured capacitances not related to the desired input, as well as, sensitivity variations with changing environmental conditions. Some fraction of the fields causing the unwanted coupling can be shorted out by a low impedance guard placed near or between the sensors to reduce these effects. The effect of the guard on the measured capacitances and sensitivity can also be reduced to the extent that it follows the voltage on the sensor (and minimizes charge transfer), which distinguishes it from a simple (e.g. ground) shield. In the embodiment shown inFIG. 5 ,individual sensing electrodes 501A-B are used to capacitively detect the presence of an object and thus provide their respective measurable capacitances. During operation, a guarding signal is provided on the guardingelectrode 503 using alow impedance path 507. The guarding signal helps to shield the sensor electrodes 501 from unintended coupling with the environment and helps to reduce the net charge transferred from the guardingelectrode 503 onto integratingcapacitances 510A-B during the course of the charge transfer processes. During a portion of the applying of the pre-determined voltage, the guard signal can apply a voltage to the guardingelectrode 503 approximately equal to the voltage applied to the predetermined voltage. Then, before the charge sharing between the active sensing electrode (e.g. 501A-B) with its associated integrating capacitance (e.g. 510A-B) ends, the voltage of the guard signal may be changed to be approximately equal to the voltage on the associated integrating capacitance (e.g. 510A-B). If a constant voltage is chosen to approximate the voltage on the associated integrating capacitance (an approximation since the voltage on the integrating capacitance changes during and between repetitions of the charge transfer process), the voltage applied to the guardingelectrode 503 may be changed to a voltage between the appropriate threshold voltage (VTH) and the voltage on the associated integrating capacitance 510 after reset. The absolute values of the voltages of the guard signal are less important than the voltage swing (i.e. change in voltage) of the guard signal. For example, an offset between the guarding electrode voltage and the sensing electrode voltage would not affect the usefulness of the guard, since for charge transfer through a capacitance, the voltage swing (i.e. change in voltage) is important and the absolute voltage values are not. - These guarding voltages of the guard signal may be generated in any manner, such as by tying the guarding
electrode 503 to a guarding voltage generator circuit of any sort. In the exemplary embodiment shown inFIG. 5 , animpedance divider circuit 505 suitably produces at least two different values of voltages depending upon the signal applied by I/O 508 ofcontroller 502 and the types and values chosen for the components comprisingimpedance divider circuit 505. Specifically, animpedance divider 505 composed of a resistive voltage divider can be used. With such a voltage divider, if the signal from I/O 508 is +VDD or if the I/O 508 is held at high impedance, the guarding voltage is +VDD. Alternatively, if the signal from I/O 508 is ground, the guarding voltage is a predetermined fraction of VDD such as (+VDD)/2. Thus, the guarding signal can be made to comprise a first and second reference voltage that can be selected using the I/O 508. Alternatively, the guarding signal can switch between a reference voltage and an open circuit condition. - As another example, the guarding voltage of the guard signal is changed between the application of the charging pulses that apply the pre-determined voltage and the subsequent sharing period. The guarding voltage swing of the guard signal can also change between repetitions of the charge transfer process, such as between a reset and the last measurement used in a determination of the measurable capacitance. Alternate embodiments could implement impedance divider circuits with one or more resistances, inductances, or capacitances for ease of design, ease of production, more effective guard signals, and the like. Digital-to-analog converters, pulse-width modulators, and the like can also be used to generate the guarding signal, or active buffers (voltage followers) may be used to lower their impedance. The various charge transfer sensing techniques described herein, coupled with the ease of multi-channel integration, provide for highly efficient application of shared guard signals.
- It should be noted that while the embodiments illustrated in
FIGS. 3-5 each illustrate a different type of component sharing, that in other embodiments the different types of component sharing can be combined. For example, in some embodiments both the passive network and the voltage conditioning circuit may be shared, while in other embodiments all three types of components may be shared. Furthermore, the various types of sharing can be implemented to share components over different combinations of sensing channels. For example, the passive network and integrating capacitances can be implemented to be shared between four channels, while a voltage conditioning circuit is implemented to be shared over twenty channels, and while the guarding electrode is implemented to be shared over all electrodes. Similarly, an I/O used to reset or reference the integrating capacitor might be shared between multiple sensors or even used in different ways at different times as inFIG. 14 discussed below. Thus, different levels of sharing can be implemented for the different components in the same sensing device. Again, these are just some example of the types of ways in which component sharing can be provided. - More detailed examples of how a charge transfer process can be performed to measure capacitance with the embodiments illustrated in
FIGS. 3, 4 and 5 will now be given. Turning now toFIG. 6 , anexemplary timing scheme 600 illustrates the voltages associated with the measurable capacitances and integrating capacitances. Specifically,FIG. 6 includes atrace 630 which shows an exemplary set of charging voltage pulses 610 that can be provided to a measurable capacitance using a corresponding I/O (e.g. I/O 304A formeasurable capacitance 312A inFIG. 3 ). The charging voltage pulses 610 include both logic low (0) output portions 609 and logic high (1) output portions 601. The logic high output portions apply the pre-determined voltage to the measurable capacitance. The logic high portions 601 of charging voltage pulses 610 are generally selected to have a period shorter than the response time of the RC circuit that includes passive impedance and integrating capacitance such that any charge leakage to the integrating capacitance during the applying of the pre-determined voltage will be negligible, although applying an opposing voltage can compensate for this leakage. - Logic low output portions 609 provide an “opposing” voltage that precedes the logic high output portions 601 that apply the pre-determined voltage. The “opposing” voltage has a magnitude opposite that of the pre-determined voltage and helps compensate for current leaking through the passive impedance during the charge transfer process by “current cancelling” without reducing the signal created by the shared charge. The durations of the logic output portions 609 are suitably chosen so the amount of parasitic charge removed by driving the opposing voltage is mostly equal to the amount of parasitic charge added by applying the pre-determined voltage. The opposing voltage might be only applied during some of the repetitions of the sharing during a cycle, and might be dependent on the elements used to determine the measurable capacitance (e.g. number of repetitions, accumulated charge on integrating capacitance, etc.).
- Between the applications of the charging voltage pulses 610, the measurable capacitance is allowed to share charge with the integrating capacitance by holding the associated I/O to a high impedance state (Z) 612. The embodiment of
timing chart 600 shows the pre-determined voltage being applied by logic highs, such that the measurable capacitance charges during the applying of the pre-determined voltage and discharges through the passive impedance to the integrating capacitance during charge sharing. Theexemplary voltage trace 634 shows the resulting voltage at the measurable capacitance, andexemplary voltage trace 636 shows the resulting voltage at the integrating capacitor. Specifically,voltage trace 634 illustrates how each opposing voltage portion 609 of a charging pulse 610 drives the voltage on measurable capacitance to the opposing voltage, how each pre-determined voltage portion 601 of a charging pulse drives the voltage on measurable capacitance to the pre-determined voltage (which is shown as +VDD inFIG. 6 ).Traces -
Voltage trace 636 exhibits slight drops and rises in response to the driving of the opposing voltage and the pre-determined voltage in “current cancelling.”Voltage trace 636 also shows the gradual increase in voltage at the integrating capacitance as the integrating capacitance accumulates charge shared by the measurable capacitance over repeated charge transfer processes. - Voltage traces 634 and 636 both show how the measurable capacitance is charged relatively quickly with each application of the pre-determined voltage through the associated I/O, while the relatively longer time constant of the measurable capacitance and passive impedance that forms the passive network causes a relatively slower sharing with the integrating capacitance. At the same time other measurable capacitances may be isolated from the application by the low impedance of the integrating capacitance and longer time constant of it with the passive impedance. Where the reset of the integrating
capacitance 614 between measurement cycles takes place by driving through the same passive impedance the time constant is also longer since the integrating capacitance is larger. - Several different methods can be used to determine the measurable capacitance from the voltage at the integrating capacitance. In one method, the voltage at the integrating capacitance is compared to an appropriate threshold voltage (VTH) to provide a single bit analog-to-digital (A/D) conversion of the voltage on the integrating capacitance. It should again be noted that the voltage at the integrating capacitance does not have to be measured directly. Instead, the voltage at the measurable capacitance may be used as a representation of the charge on integrating capacitance when there is no I/O directly coupled to the integrating capacitance (See
FIGS. 3 and 5 for two examples). This voltage comparison can be performed by a comparator function implemented in the I/O of the controller. In this case, the threshold VTH can be the predetermined threshold of the digital input buffer (such as a CMOS or TTL threshold). Thus, an I/O can be used to compare the voltage at the integrating capacitance to the threshold voltage and thus obtain a representation of the charge on integrating capacitance. In one embodiment, this threshold voltage is roughly equivalent to the midpoint between the high and low logic values. - The embodiment shown in
FIG. 6 uses a comparator-type method, where the charge transfer process of applying charging pulses to the measurable capacitance and allowing charge sharing by the measurable capacitance with the integrating capacitance repeats until the voltage at the integrating capacitance is detected to exceed the threshold voltage VTH. The I/O of the controller has input functionality, and can be read to measure the voltage by comparing it with an input threshold. This measuring and ascertaining of the voltage on the integrating capacitance can occur for some each or a specified portion of the charge transfer process. - After the voltage at the integrating capacitance exceeds the threshold (indicated by
points 603A-B on trace 636), reset signals 614 can be applied to the integrating capacitance to reset the charge on the integrating capacitance. Although sometimes only a few performances of the charge transfer process are needed to cross the threshold, typically a hundred or more performances (thousands) of the charge transfer process are involved.Trace 636 shows the threshold being passed after only four performances of the charge transfer process for convenience of explanation not as a typical case. By ascertaining the number of charge transfer processes performed until the voltage on the integrating capacitance exceeds the threshold voltage VTH, a value of the measurable capacitance can be effectively determined. That is, the number of charge/share repetitions performed for a measurable capacitance to produce a known amount of accumulated charge on the integrating capacitance can be effectively correlated to the actual capacitance of measurable capacitance. - This comparator-type method can also be implemented with any combination of circuitry internal and/or external to the controller as appropriate. Many variations of this comparator-type method also exist and are contemplated. For example, multiple thresholds can be provided using a multitude of reference voltages for one or more comparators, using a specialized input of the controller, or using an input of the controller having hysteresis (e.g. a Schmitt trigger type input). If multiple thresholds are used, the charge transfer process can also change as different thresholds are reached. For example, the charge transfer process can be configured to transfer relatively larger amounts of charge (e.g. by multiple charging and sharing cycles) to reach coarser thresholds if the thresholds are not evenly spaced, or first thresholds if there is a multitude of thresholds that all must be crossed. The number of performances (or repetitions) of each type of charge transfer process needed to cross the last threshold crossed can be used to determine the value of the measurable capacitance, and additional information concerning the crossing of other thresholds can also be used to refine the determination.
- In other embodiments, alternative methods of determining the value of the measurable capacitance are used. For example, in one embodiment, a direct multi-bit measurement of the voltage on the integrating capacitance is taken and used to determine the capacitance of the measurable capacitance. For example, the controller can include a high resolution analog-to-digital function that allows more accurate measurement of the voltage at an I/O. In such embodiments, the charge transfer process can execute for a pre-set number of times, after which a multi-bit value of the voltage is measured. After the pre-set number of repetitions and the measurement, the charge on the integrating capacitance can be reset for the next cycle of executions of the charge transfer processes.
- Other variations on these methods are possible. For example, multi-bit measurements can be taken at multiple times in a set of charge transfer processes performed between resets. As another example, the number of executions of the charge transfer process does not have to be pre-set, such that both the number of charge transfer processes performed and the voltage at the integrating capacitance are tracked to produce a value of the measurable capacitance.
- Other methods can be used to determine the measurable capacitance from the voltage (which is a function of the accumulated charge) on the integrating capacitance. For example, a dual slope method entails performing the charge transfer process for a pre-set number of times, and then drawing charge from the integrating capacitance using a current source or a discharge circuit with a known time response, such as a first order response. The time needed for the charge on integrating capacitance to fall to a known value such as zero can be monitored and quantized to produce a single or a multi-bit measurement of the representation of the charge on the integrating capacitance. This measurement can be used along with the pre-set number in determining the value of measurable capacitance. With such methods, the integrating capacitance may be left at the known value after measurement, such that no separate reset signal needs to be applied.
- Other changes could be made to the basic structures and operations described above. For example, while the timing scheme shown in
FIG. 6 assumes a “positive” transfer of charge from measurable capacitance to the integrating capacitance, whereas equivalent embodiments could be based upon sharing of charge in the opposite direction. That is, charge could be placed on integrating capacitance during reset, and this charge then removed and drawn through the passive impedance to the measurable capacitance during sharing. - As other examples, an “oscillator” type method can use two different pre-determined voltages to charge measurable capacitance and two threshold voltages (e.g. Vdd/3 and 2Vdd/3) to determine measurable capacitance. Turning now to
FIG. 7 , an “oscillator” type method is illustrated in voltage traces 710, 703, 706 and 730. In the embodiment shown inFIG. 7 , both crossings of the threshold voltages VTH1 and VTH2 are measured and can be used along with the number of times the charge transfer process was performed to determine the value of measurable capacitance. Specifically, thetrace 710 shows an exemplary set of charging voltage pulses 711 and 712 that can be provided to a measurable capacitance using a corresponding I/O. Trace 703 shows the resulting charge on the measurable capacitance whiletrace 706 shows the result of that charge sharing on the integrating capacitance. - In the illustrated embodiment, the charging pulses 711 in
period 701 add charge to the measurable capacitance, while the charging pulses 712 inperiod 702 remove charge from the measurable capacitance. With its cycles of both “positive” and “negative” charging, this oscillator embodiment does not need a separate reset signal after each measurement cycle as described above. This is because the amplitude and rate of change (amount of voltage change per performance of the charge transfer process) of the voltage waveform that results on the integrating capacitance between the voltage thresholds is roughly independent of the starting voltage on the integrating capacitance (assuming the thresholds and the measured capacitance are roughly constant) and are indicative of the value of the measurable capacitance used in the charge transfer process. A larger value of measurable capacitance means that the voltage thresholds VTH1 and VTH2 will be crossed with fewer performances of the charge transfer process, and a smaller value of measurable capacitance means that the voltage thresholds VTH1 and VTH2 will be crossed with more performances of the charge transfer process. Therefore, it is not required that the integrating capacitance be reset or otherwise placed at a known state after a threshold is crossed and before beginning a set of charge transfer processes (although it will certainly work with resets, and it could be done with a single threshold). - Because the repetition measurement can take place between the voltage thresholds for cycles of both “positive” and “negative” charging, additional charge transfer processes can be performed after a threshold voltage is crossed without detrimentally affecting the sensor's performance, even if no reset of the integrating capacitance occurs. Similarly, the sensitivity of sum of the total number of cycles of “positive” and “negative” charging will be significantly reduced for constant current leakage or to capacitively coupled interference of significantly lower frequency (compared to the oscillation). In an alternate embodiment the number of charging cycles in both directions could be constant (thought they could be different or adjusted between measurements) and the upper and lower voltage thresholds (e.g. VTH1, VTH2 in the case of the embodiment of
FIG. 7 ) could be variable. That is a multi-bit quantized measurement of the voltage near either end of the “positive” and “negative” charging cycles could be made rather than counting the cycles similar to that described in the charge and reset method above. The quantized size of the swings could then be digitally filtered, and a variety of methods (other than a simple reset) could be used to center the swings on measurement range to improve dynamic range (including simply more charging cycles for some sensors in one direction or another). - Even though the circuits of the present invention can be driven such that the charge transfer processes end based on when the applicable threshold voltage (e.g. VTH1, VTH2 in the case of the embodiment of
FIG. 7 ) is crossed, that mode of operation may not be desirable in many embodiments with multiple sensing channels. Instead, it may be preferable to continue performing charge transfer processes after the crossing of the applicable threshold voltage in a multiple sensing channel system where the channels are operated concurrently (e.g. all in parallel). The total number of charge transfer processes performed may then be based on when the last-in-time filter capacitance of the multiple sensing channels crosses its threshold (or even later). The total number of charge transfer processes can also be pre-selected to be a large enough number that (at least in most cases) the last-in-time filter capacitance of the multiple sensing channels would have crossed its threshold. - The number of performances (or repetitions) of the charge transfer process during
time period 701 and the number of performances of the other charge transfer process duringtime period 702 can be determined in any manner. In various embodiments, the I/O coupled to the measurable capacitance incorporates an input having hysteresis, such as Schmitt trigger feature, that provides the two threshold/comparison voltages VTH1 and VTH2. The I/O can thus be used to read the voltage at times indicated bypoints 730 ofFIG. 7 . As the voltage on integrating capacitance is sensed to have passed higher threshold VTH1 (e.g. atpoint 730C) in such embodiments, a set of charge transfer processes can be applied in the opposing direction to reduce the voltage on an integrating capacitance. Similarly, as the voltage on integrating capacitance is sensed to have passed lower threshold VTH2 (e.g. atpoint 730E) another set of charge transfer process can be applied in the opposing direction to increase the voltage on integrating capacitance. As shown bytrace 706, the sensing scheme produces a voltage at the integrating capacitor that approaches thresholds from the correct direction such that hysteretic inputs such as Schmitt trigger inputs will function correctly in the system and provide the appropriate thresholds for the periods of charge transfer processes. An example of a circuit appropriate for this sensing method isFIG. 4B , since there is more than one voltage threshold, it would be appropriate to use an additional I/O to change the voltage ratio for each threshold on theconditioning circuit 475. - In still other embodiments, pre-selected numbers of performances of the charge transfer process are combined with the use of thresholds. That is, the charging and/or discharging processes may execute for a pre-determined or pre-established number of cycles, but the charge transfer process in which the voltage on integrating capacitance crosses a threshold voltage is identified.
FIG. 7 , for example, shows fourteen charge transfer processes and sixmeasurements 730A-F (again, this number of transfers is only for demonstration purposes and smaller than would be typically used), with the time that eachmeasurement 730A-F is taken illustrated by an arrow. In the illustrated example, seven charge transfer processes and three measurements are performed during each charging or discharging cycle even though threshold voltage VTH1 is crossed atpoint 715, just prior to the third sample indicated bypoint 730C and threshold VTH2 is crossed atpoint 717, just prior to the second sample indicated bypoint 730E. A third sample is still taken, as indicated bypoint 730F, and a seventh charge transfer process is still performed even though the second sample indicated that the voltage had already crossed threshold VTH2. However, such additional samples may provide an added advantage, particularly when multiple sensing channels are measured using a common integrating capacitance, in that the slope direction is relatively constant across channels, even though the measured capacitance may vary from channel to channel. Such embodiments may also provide other advantages such as in improved rejection of incorrect readings of a threshold voltage having been crossed. - It should also be noted that the
measurements 730A-F are shown as taken during later parts of eachtime period -
FIG. 7 shows this additional optional feature in that the pulses 711 and 712 used to charge and discharge measurable capacitance need not be equally spaced in time, and are more frequent earlier in theperiods FIG. 7 , the charging and discharging pulses applied to measurable capacitance are initially applied fairly rapidly to speed the sensing process, while later the pulses are applied more slowly to ensure complete sharing of the charge on the measurable capacitance and sufficient time for accurate measurement. In other embodiments, the measurement period may be faster than the non-measurement period, as appropriate. Charging and discharging pulses 711, 712 (or any other charging pulse) can also vary in timing for other reasons, and they need not be equally spaced in time or be of equal duration. - As discussed above, another capacitance measurement technique that can be used in the embodiments of the invention is referred to generally as “sigma delta” (or delta sigma). In general, the term sigma delta relates to capacitance detection method that incorporates summation (sigma) and difference (delta) of electrical charge to quantify an electrical effect that is exhibited by an electrode or other electrical node. Typically, an integrating capacitance accumulates charge transferred from the measurable capacitance from multiple charge sharing events. Additional electrical charge having an opposing sign to the charge received from the measurable capacitance is also applied in quantized pre-set quantities to maintain the integrated charge near a known level (most generally quantized charges of both signs may be used). That is, a quantized amount of charge is appropriately subtracted or added from the integrating capacitance to maintain the filter output near the desired level. By digitally filtering (e.g. by averaging, summation, higher order finite impulse response, infinite impulse response, or Kaiser filters, etc.) the quantized charge(s) applied to the integrator, the amount of charge transferred by the measurable capacitance can be ascertained. This capacitance value, in turn, can be used to identify the presence or absence of a human finger, stylus or other object in proximity to the sensed node in a proximity sensor device, and/or for any other purpose.
- Turning now
FIG. 8 , an embodiment of acapacitive sensor 800 is illustrated. In general, thecapacitive sensor 800 uses sigma-delta measurement techniques implemented in a controller to measure capacitance, and shares components to reduce device complexity and improve performance. Thecapacitive sensor 800 includes apassive network 809 coupled to themeasurable capacitances 812A-B, thepassive network 809 including an integratingcapacitance 820 configured to store charge received from themeasurable capacitances 812A-B. Additionally, thecapacitive sensor 800 includes acharge changing circuit 828 coupled topassive network 809, where the charge changing circuit comprises adelta capacitance 826. Note that simple charge changing circuits typically comprise one or more switches (e.g. I/Os) and one or more passive components (e.g. capacitor, resistor, etc.). Note that timed current sources, switched capacitor charge pumps and a variety of other more complex methods could also be used to generate quantized charge deltas (e.g. an additional I/O and passive impedance along with a differently sized CD could provide more differently quantized amounts of charge changing). Acontroller 802 includes two I/Os measurable capacitances capacitive sensor 800, a voltage is repeatedly applied to a selected measurable capacitance 812 using a corresponding I/O, and charge is repeatedly shared between the selected measurable capacitance and thepassive network 809 to accumulate charge on the integratingcapacitance 820. Furthermore, the charge on the integratingcapacitance 820 is repeatedly changed by a quantized amount of charge using thedelta capacitance 826, with this charge changing responsive to the charge on the integratingcapacitance 820 being past a threshold level. The results of the charge threshold detection are a series of quantized measurements of the charge on the integratingcapacitance 820, which can be filtered to yield a measure of the measurable capacitance 812. - In the illustrated embodiment, the
passive network 809 in general, and the integratingcapacitance 820 specifically, are shared between two sensing channels, where each of the sensing channels corresponds to one of themeasurable capacitances 812A-B. Furthermore, thecharge changing circuit 828 generally including I/O 808 anddelta capacitor 826 specifically is likewise shared between the two sensing channels. It should be noted that for thepassive network 809 to be considered as being shared by multiple sensing channels it is not required that all elements of thepassive network 809 be shared. Instead, it is sufficient that one element (e.g., the integrating capacitance 820) be shared between channels. Nor is it required that thepassive network 809 be shared between all channels on the capacitive sensor. Instead, it is sufficient that thepassive network 809 be shared between any of the channels on the capacitive sensor. Similarly, an I/O used for measurement (quantization) of the charge on the integratingcapacitor 820 may be made by a shared I/O 808, by the unshared I/Os measurable capacitances 812A-B respectively, or a variety of other combinations. - As stated above, the
capacitive sensor 800 includes apassive network 809 and a delta capacitance (CD) 826 to charge and discharge into an integrating capacitance (CI) 808 as appropriate. Also, in this embodiment,passive network 809 is implemented with an integratingcapacitance 808 and passive impedances 810, 812. The passive impedances (including 806) server to transfer charge applied to thecapacitances 812A-B and 826 while isolating the integrating capacitance from short charging pulses on the I/Os capacitance 808 is shown implemented with a conventional capacitor configured as an imperfect integrator. The integrating capacitance has a capacitance that is larger, and often significantly larger (e.g. by one or more orders of magnitude), than the value of thedelta capacitance 826 and the expected value ofmeasurable capacitances 812A-B. In various embodiments, for example,measurable capacitances 812A-B anddelta capacitance 826 may be on the order of picofarads while the integratingcapacitance 808 is on the order of nanofarads, although other embodiments may incorporate widely different values for the particular capacitances. The integrating capacitance could comprise multiple capacitances 820A&B and similar to integrating capacitances 310A&B inFIG. 3B provide voltage conditioning to the integrating capacitance. - It should be noted that while in the illustrated embodiment, the
passive network 809 includes the integratingcapacitance 820 and the passive impedances 810 and 812 that this is just one example implementation. In other embodiments incorporating additional switches, thepassive network 809 is simply an integratingcapacitance 820, which can be a single capacitor. Alternatively, thepassive network 809 may contain any number of resistors, capacitors and/or other passive elements as appropriate, and a number of examples of passive networks are described below -
Measurable capacitances 812A-B are typically the capacitance created by sensor electrodes, where the capacitance varies according to the proximity of objects to the electrode. For sensor devices that measure input from one or more fingers, styli, and/or other stimuli, measurable capacitance 812 often represents the total effective capacitance from a sensing node to the local ground of the system (“absolute capacitance”). The total effective capacitance for input devices can be quite complex, involving capacitances, resistances, and inductances in series and in parallel as determined by the sensor design and the operating environment. For some sensor constructions where electrodes are constructed of high resistance conductors such as ITO or carbon ink the resistances could be significant. Resistances may be added between distributed measurable capacitances connected to separate integrating capacitances such that the proportion of charge transferred to integrating capacitance is in inverse proportion to that resistance. In other cases, measurable capacitance 812 may represent the total effective capacitance from a driving (transmitting) node to a sensing node (generally referred to as “transcapacitance”). This total effective capacitance can also be quite complex. In either case, a charging voltage referenced to the local system ground can be initially applied to the measurable capacitance 812, as described more fully below, and measurable capacitance 812 is then allowed to share charge with thepassive network 809. - In one operating technique a charge transfer process allows charge from measurable capacitance 812 to be transferred to integrating
capacitance 820, and for opposing charge fromdelta capacitance 826 to adjust the level of charge held by integratingcapacitance 820. When the voltage at I/O 808 is below a threshold voltage thedelta capacitance 826 is charged, and the integratingcapacitance 820 coupled to thedelta capacitance 826 bypassive impedance 806 shares charge with it. A quantized charge fromdelta capacitance 826 is transferred to integratingcapacitance 820, thereby producing a change in voltage at the integratingcapacitance 808 measurable at I/O 808 to move it towards a measurement threshold (though it could also be driven past it). After an initial startup period, the voltage will be driven to approximate the comparator voltage by this negative feedback which results in charge being added to or subtracted from the integratingcapacitance 808 by thedelta capacitance 826 to balance the charge shared from the measurable capacitance. This process may continue with or without charge transfer frommeasurable capacitances 812A-B - It should be noted that in the illustrated embodiment, no direct action is required to share charge between measurable capacitances 812 and the integrating
capacitance 808. Instead, only a pause with sufficient time to allow transfer is needed. It should also be noted that although the measurable capacitances 812 may be statically coupled to the integratingcapacitance 820, charge sharing between capacitances can be considered to substantially begin when the charging step ends (e.g., when the applying of voltage to the measurable capacitance ends). Furthermore, the charge sharing between capacitances can be considered to substantially end when the voltages at the capacitances are similar enough that negligible charge is being shared. Charge sharing can also substantially end with the next application of a voltage because the low impedance charging voltage being applied dominates. Thus, even in a passive sharing system where the filter capacitance is always coupled to the measurable capacitance, the low impedance of the applied voltage source makes the charge on the measurable capacitance that would be shared negligible until the applied voltage is removed. Clearly, active switches may also be used in place of the passive impedances, as well. - When charge from measurable capacitance 812 is effectively transferred to the
passive network 809, the charge on thepassive network 809 is appropriately measured and changed if the amount of charge is determined to be past a suitable threshold value. Charge measurement may take place in any manner. In various embodiments, the voltage onpassive network 809 representative of that charge is obtained from an input/output (I/O) pin (e.g. 808) of a microcontroller or other device. In a simple embodiment, a CMOS (or TTL) digital input acts as a comparator (1-bit quantizer) with a reference voltage equal to the threshold level of the digital input. - In the embodiment illustrated in
FIG. 8 , the integratingcapacitance 820 is not directly connected to an I/O of the controller. However, the voltage at an I/O 808 can be used as the representation of the charge on integratingcapacitance 820, and thus the charge on the integratingcapacitance 820 can still be effectively measured at I/O 808 by sharing the quantizer. For example, the voltage at I/O 808 can be compared to a threshold voltage VTH. This threshold voltage could be ˜VDD/2 fro a CMOS input, or a variety of other levels, and alternately, the voltage could be measured at I/O - As the charge on the
passive network 809 passes an appropriate threshold value, a “delta” charge that opposes the charge shared from the measurable capacitance 812 is applied (e.g. via delta capacitance 826) to change the charge on thepassive network 809. In this manner, the charge onpassive network 809 can be maintained to what is needed for the associated voltage onpassive network 809 to approximately equal the threshold value (VTH), if the measurable capacitance 812 is within range. That is, because of negative feedback, the voltage across (and thus the charge on) the integratingcapacitance 808 remains approximately constant during operation due to the control loop. - The voltage application, charge transfer, charge changing and/or other steps may be individually and/or collectively repeated any number of times to implement a number of useful features. For example, by obtaining multiple quantized values of measurable capacitance 812, the measured values can be readily decimated, filtered, averaged, differenced, and/or otherwise digitally processed within the control circuitry to reduce the effects of noise, to provide increasingly reliable measurement values, and/or the like. By changing the charge multiple times the size and quantization of the charge changing network can be varied. By varying the timing of the application of voltage to the measurable capacitance the sampling frequency of the sensor can be varied to improve noise performance.
- Turning now
FIG. 9A , another embodiment of acapacitive sensor 900 is illustrated. In this embodiment thecapacitive sensor 900 shares avoltage conditioning circuit 925 that is adapted to provide a variable reference voltage. For example, a variable reference voltage that varies with a threshold voltage of thecontroller 902. This embodiment does not, however, share a passive network or an integrating capacitance (although it could by adding additional passive networks to share either integrating capacitance), and instead uses a plurality of integratingcapacitances 910. - Like
sensor 800, thecapacitive sensor 900 uses sigma-delta measurement techniques implemented in a controller to measure capacitance, and shares components to reduce device complexity and improve performance. Thecapacitive sensor 900 includes two passive networks coupled to themeasurable capacitances 912A-B, with each passive network an integratingcapacitance 910 configured to store charge received from themeasurable capacitances 912A-B, and respectivepassive impedance 905A-B. Additionally, thecapacitive sensor 900 includes charge changing circuits coupled to passive networks, where each charge changing circuit comprises adelta capacitance 826 and another passive impedance 928. - A
controller 902 includes two I/Os 904 coupled to the measurable capacitances 912, two I/Os 906 coupled to he integratingcapacitances 910, and two I/Os coupled to the delta capacitances 926. During operation ofcapacitive sensor 900, a voltage is repeatedly applied to a selected measurable capacitance 912 using a corresponding I/O, and charge is repeatedly shared between the selected measurable capacitance 912 and the integratingcapacitance 910 to accumulate charge on the integratingcapacitance 910. Furthermore, the charge on the integratingcapacitance 910 is repeatedly changed by a quantized amount of charge using the delta capacitance 926, with this charge changing responsive to the charge on the integratingcapacitance 910 being past a threshold level. The results of the charge threshold detection are a quantized measurement of the charge on the integratingcapacitance 910, which can be filtered to yield a measure of the corresponding measurable capacitance 912. It should be noted that the two sensing channels corresponding tomeasurable capacitances 912A-B respectively, may be operated concurrently, sequentially, or in a variety ways. - In the illustrated embodiment, a
voltage conditioning circuit 925 is shared between the twomeasurable capacitances 912A-B. Thevoltage conditioning circuit 925 includes afirst impedance 927 and asecond impedance 929 coupled as an impedance divider (e.g. a voltage divider) between VDD and ground. As such, thevoltage conditioning circuit 925 provides a variable reference voltage tonode 907. In one specific embodiment, thevoltage conditioning circuit 925 provides a voltage that varies with a threshold voltage of thecontroller 902. - Specifically, the
voltage conditioning circuit 925 is coupled to the side of integratingcapacitances 910 opposite the measurable capacitances 912, and to either or both power supply rails (coupling to +VDD and ground) associated with the implementation ofcapacitance sensor 900. With the configuration shown inFIG. 9 , fluctuations in the supply rails (also “supply voltage ripple”) induce similar fluctuations in the voltage atnode 907, and therefore can be used to compensate for fluctuations in thresholds associated withcontroller 902 induced by the same supply voltage ripple. - For
compensation circuit 925 shown inFIG. 9 , the impedance divider can be a voltage divider formed from two resistances or two capacitances coupled to +VDD and ground. The impedance divider ofcircuit 925 has a “common node” coupled to the integratingcapacitances 910A-B atnode 907. Resistive versions ofimpedances impedances impedances capacitances 910 can be biased toward any voltage that lies between the two supply voltages. Moreover, variations in supply voltage will be automatically compensated by the voltage conditioning (compensating)circuit 925. This is because such a voltage divider provides a voltage that reflects the fluctuations in a power supply voltage ratiometrically with appropriate lag relative to the compensated threshold variation. - Of course, this is just one example of the type of voltage conditioning circuit that can be shared between measurable capacitances in a capacitive sensor. It should also be noted that while the
voltage conditioning circuit 925 is illustrated as being shared between twomeasurable capacitances 912A-B, that this concept can be expanded to share thevoltage conditioning circuit 925 between more measurable capacitances to further enhance device efficiency. - Turning now to
FIG. 9B , another embodiment of acapacitive sensor 900 is illustrated. Like thesensor 900, the capacitive sensor 950 includes avoltage conditioning circuit 951 that shared between the twomeasurable capacitances 912A-B. Thevoltage conditioning circuit 951 includes afirst impedance 927 and asecond impedance 929 coupled as a voltage divider between VDD and ground. The voltage conditioning circuit also includes I/O 952 coupled tonode 907 through an I/O impedance 953. Like thevoltage conditioning circuit 925 ofFIG. 9A , thevoltage conditioning circuit 951 provides a variable reference voltage tonode 907 that varies with a threshold voltage of thecontroller 902 - Additionally, the I/
O 952 and I/O impedance 953 allows the compensated voltage that is applied tonode 907 to be dynamically changed responsive to a change in the output of I/O 952. For example, if the afirst impedance 927, thesecond impedance 929, and the I/O impedance 953 all have the same value, then the compensated voltage provided tonode 907 will be approximately ⅔ VDD when I/O 952 is driven high, and will be approximately ⅓ VDD when I/O 952 is driven low. This can be useful in applications where I/O thresholds may change (e.g. Schmitt inputs with hysteresis) or to move the voltage on the measurement I/O away from a threshold during part of the measurement cycle. - Furthermore, the I/
O 952 can serve as a reference for the other I/Os. Specifically, because the changes in the threshold voltage of the measuring I/Os O 952, the use of I/O 952 can be used to control the voltage onnode 907 and thus compensate for changes in other threshold voltages. This means that variations in the threshold voltage of I/Os O 952 sufficiently closely. - Turning now to
FIG. 9C , another embodiment of acapacitive sensor 975 is illustrated. In this embodiment, instead of including a separate voltage conditioning circuit (e.g.,circuit 925 ofFIG. 9A andcircuit 951 ofFIG. 9B ) shared voltage conditioning for onemeasurable capacitance 912A-B is provided by the passive impedances and I/Os corresponding to the othermeasurable capacitance 912A-B. Specifically, by driving I/O 903A to VDD and driving I/O 904A low thenode 911A is driven to ½ VDD. Likewise, by driving I/O 903B to VDD and driving I/O 904B low thenode 911B is driven to ½ VDD. It should be noted that in this embodiment the integratingcapacitance 910 is shared between bothmeasurable capacitances 912A-B. Thus, themeasurable capacitances 912A-B are configured to be measured alternately with the passive impedances and I/Os of one side (e.g. corresponding tomeasurable capacitance 912B) providing the voltage conditioning circuit for the other (e.g. corresponding tomeasurable capacitance 912A). Thus, the different elements of a sensor can be shared with other sensors even if the elements server different purposes at different times. - Again, the embodiment illustrated in
FIG. 9 is just one example of how shared components can be used in a capacitance sensor. Turning now toFIG. 10A , anotherembodiment capacitive sensor 1000 is illustrated. In this embodiment thecapacitive sensor 1000 shares a guardingelectrode 1002 that is provided a guard signal during at least a portion of the time in which voltage is applied to sensing electrodes 1001 and during at least a portion of the time in which the electrodes share charge with an integrating capacitance in a passive network. This embodiment does not, however, share an integrating capacitance, and instead uses a plurality of integrating capacitances 1001. - Like the embodiment illustrated in
FIGS. 8-9 , thecapacitive sensor 1000 is adapted to use sigma-delta measurement techniques implemented in a controller to measure capacitance, and shares components to reduce device complexity and improve performance. Thecapacitive sensor 1000 is coupled to twoelectrodes 1001A-B that provide measurable capacitances that vary with the proximity of objects to theelectrodes 1001A-B. Additionally, the capacitive sensor 1001 includes two passive networks 1009, with each passive network 1009 including a passive impedance 1007 and an integrating capacitance 1008 configured to store charge received from the electrodes 1001. Furthermore, the capacitive sensor includes two delta capacitances 1026 adapted to selectively change charge on the passive networks 1009. - The
controller 1002 includes two I/Os 1006A-B coupled to theelectrodes 1001A-B and to thepassive networks 1009A-B, and two I/Os 1004A-B coupled to the delta capacitances 1026. During operation ofcapacitive sensor 1000, a voltage is repeatedly applied to an electrode 1001 using a corresponding I/O 1006, and charge is repeatedly shared between the selected electrode 1001 and the passive network 1009 to accumulate charge on the integrating capacitance 1008. The voltage on the integrating capacitance can be measured by a threshold of I/O 1006 as representative of the accumulated charge on 1008 through impedance 1007 relative to ground through passive impedance 1007 (although I/O 1004 might also be similarly used). Furthermore, the charge on the integrating capacitance 1008 is repeatedly changed by a quantized amount of charge using the charge changing circuit comprising delta capacitance 1026 and I/O 1004, with this charge changing responsive to the charge on the integrating capacitance 1008 being past a threshold level. The results of the charge threshold detection are a quantized measurement of the charge on the integrating capacitance 1008, which can be filtered to yield a measure of the corresponding measurable capacitance created by the electrode 1001. - This embodiment also includes a guarding
electrode 1003 that serves to shield the sensor electrodes 1001 from unintended electrical coupling. In the embodiment shown inFIG. 10 ,individual sensing electrodes 1001A-B are used to capacitively detect the presence of an object and thus provide their respective measurable capacitances. During operation, a guarding signal is provided on the guardingelectrode 1003 using I/O 1011. The guarding signal helps to shield the sensor electrodes 1001 from unintended coupling with the environment and helps to reduce the net charge transferred from the guardingelectrode 1003 onto integrating capacitances 1008 during the course of the charge transfer processes. During a portion of the applying of the pre-determined voltage, the guard signal can apply a voltage to theguarding electrode 1003 approximately equal to the voltage applied to the predetermined voltage. Then, before the charge sharing between the active sensing electrode (e.g., 1001A-B) with its associated integrating capacitance (e.g. 1008A-B) ends, the voltage of the guard signal may be changed to be approximately equal to the voltage on the associated integrating capacitance (e.g. 1008A-B). A constant voltage swing may be chosen to approximate the voltage on the associated integrating capacitance 1008 since the sigma delta feedback loop will keep it near a threshold voltage (VTH). The absolute values of the voltages of the guard signal are less important than the voltage swing (i.e. change in voltage) of the guard signal. For example, an offset between the guarding electrode voltage and the sensing electrode voltage would not affect the usefulness of the guard, since for charge transfer through a capacitance, the voltage swing (i.e., change in voltage) is important and the absolute voltage values are not. - These guarding voltages of the guard signal may be generated in any manner, such as by tying the
guarding electrode 1003 to a guarding voltage generator circuit of any sort. In the exemplary embodiment shown inFIG. 10A , animpedance divider circuit 1012 suitably produces at least two different values of voltages depending upon the signal applied by I/O 1011 ofcontroller 1002 and the types and values chosen for the components comprisingimpedance divider circuit 1012. Other implementations of the guarding voltage generator might also comprise active components, andFIG. 10A shows only one simple implementation. - Specifically, an impedance divider composed of a resistive voltage divider can be used. With such a voltage divider, if the signal from I/
O 1011 is +VDD or if the I/O 1011 is held at high impedance, the guarding voltage is +VDD. Alternatively, if the signal from I/O 1011 is ground, the guarding voltage is a predetermined fraction of VDD such as (+VDD)/2. As one example, the guarding voltage of the guard signal is changed between the application of the charging pulses that apply the pre-determined voltage and the subsequent sharing period. Alternate embodiments could implement impedance divider circuits with one or more switches, resistances, inductances, or capacitances for ease of design, ease of production, more effective guard signals, and the like. - Turning now to
FIG. 10B , an alternate embodiment of acapacitive sensor 1050 is illustrated. In this alternate embodiment the guardingvoltage generator 1051 is comprised of apassive impedance 1052, acapacitor 1053, and an I/O 1054. The voltage on the guard can be controlled to ground, VDD, or to the voltage on thecapacitance 1053, which can be controlled and measured to be near a threshold of I/O 1054. Where the series impedance of 1052 and 1053 is low compared to the guarded coupled impedances (comprised of measurable capacitances, isolating impedances, etc), and where a threshold of I/O 1052 tracks other I/O thresholds this should also provide a guard voltage that tracks the threshold voltage of measurement I/Os O 1054 is turned to input, allowing the voltage to relax to a voltage that is near the reference voltage of the I/O 1054. Then, short discharging pulses can be provided to remove charge, resulting in a voltage on the guardingelectrode 1003 that closely equals the reference voltage of the measurement I/Os - Turning now to
FIG. 10C , an alternate embodiment of acapacitive sensor 1075 is illustrated. In this alternate embodiment the guardingvoltage generator 1012 is also coupled to the passive networks 1009. Specifically, the guardingvoltage generator 1012 is coupled to the integratingcapacitances 1008A and 1000B. In this embodiment, the voltage generated by thevoltage generator 1012 is used both for guarding and to provide a conditioned voltage to the integrating capacitances. - It should be noted that while the embodiments illustrated in
FIGS. 8-10 each illustrate a different type of component sharing, that in other embodiments the different types of component sharing can be combined. For example, in some embodiments both the passive network and the voltage conditioning circuit may be shared, while in other embodiments all three types of components may be shared. Furthermore, the various type of sharing can be implemented to share components over different combinations of sensing channels. For example, the passive network and integrating capacitances can be implemented to be shared between four channels, while a voltage conditioning circuit is implemented to be shared over twenty channels, and while the guarding electrode is implemented to be shared over all electrodes. Thus, different levels of sharing can be implemented for the different components in the same sensing device. Again, these are just some example of the types of ways in which component sharing can be provided. - One technique for operating a sigma delta capacitive sensor is illustrated in
FIGS. 11 and 12 . Specifically,FIG. 11 illustrates a state diagram andFIG. 12 illustrates corresponding a timing diagram, including voltage traces for the measurable capacitance VX, the integrating capacitance VI, and the delta capacitance VD, for a sensor like that shown inFIG. 10A . It should be noted that the illustrated examples are for one channel (e.g., for one measurable capacitance) and that the process would typically be repeated for each additional channel. Instate 1, the measurable capacitance is charged and the delta capacitance is cleared by driving the corresponding controller I/Os (e.g. I/O 1&2) to a known (high) logic state. Instate 2, charge is subsequently trapped on the measurable capacitance by bringing the I/O (e.g. I/O 2) coupled to the measurable capacitance to a high impedance state, and sufficient delay time is subsequently allowed for charge to share (e.g. charge or discharge) from the measurable capacitance to integrating capacitance through any isolating impedances. Insteps state 5, the changed charge is then trapped on the integrating capacitance by again setting the I/O's (e.g. I/O1) to a high impedance state. Then, instate 6, the charge on the integrating capacitance can be measured by comparing the voltage at the I/O to a reference voltage (e.g. a CMOS threshold of I/O 2). When the data has been quantized it may be stored, and the data may be summed, filtered, decimated or otherwise processed as appropriate to determine a value of the measurable capacitance. - Thus,
FIGS. 3-12 provide various embodiments of capacitive sensors that share components to improve device efficiency. However, these are just a few examples of the type of capacitive sensors that can be implemented with component sharing. For example, turning now toFIG. 13 , another embodiment of acapacitive sensor 1300 is illustrated. In the illustrated embodiment, a passive network comprising an integratingcapacitance 1310 is shared between two sensing channels, where each of the sensing channels corresponds to one of themeasurable capacitances 1312A-B. In this embodiment the twomeasurable capacitances 1312A-B are measured alternately using the shared integratingcapacitor 1310. Specifically, one side of the integratingcapacitor 1310 is driven to a low impedance (e.g. by I/O 1306A) while the other measurable capacitance on the other side (e.g.,measurable capacitance 1312B) is measured using I/Os capacitor 1310 is driven to a low impedance (e.g. by I/O 1306B) while the other measurable capacitance on the other side (e.g.,measurable capacitance 1312A) is measured using I/Os - Turning now to
FIG. 14 , another embodiment of acapacitive sensor 1400 is illustrated. In the illustrated embodiment, four integratingcapacitances 1410A-D are shared between four sensing channels, where each of the sensing channels corresponds to one of themeasurable capacitances 1412A-D. Specifically, eachmeasurable capacitance 1412A-D is measured using two of the integratingcapacitances 1410A-D. For example,measurable capacitance 1412B is measured using the two integratingcapacitances measurable capacitance 1412C is measured using the two integratingcapacitances measurable capacitances O 1404A can be driven to VDD, and the I/O 1404C can be driven to ground. Charge from themeasurable capacitance 1412B is shared with integratingcapacitors 1420A and 1410B, and the charge on those capacitors is measured using I/O 1404B. Likewise, charge from themeasurable capacitance 1412D is shared with integratingcapacitors O 1404D. - Then, to measure
measurable capacitances O 1404D can be driven to VDD, and the I/O 1404B can be driven to ground. Charge from themeasurable capacitance 1412A is shared with integratingcapacitors O 1404A. Likewise, charge from themeasurable capacitance 1412C is shared with integratingcapacitors O 1404C. - Furthermore, this arrangement provides shared voltage conditioning. Specifically, since adjacent channels are driven to VDD and ground respectively, and since the integrating capacitances 1410 and impedances 1405 are substantially equal, the measuring I/Os are naturally driven to a reference voltage of ½ VDD. As the threshold voltage of the measuring I/Os are ratiometric to VDD, changes in threshold voltage are compensated for by changes in the reference voltage ½ VDD provided to the measuring node.
- For example, when measuring
measurable capacitance 1412B, the I/O 1404A is driven to VDD and the I/O 1404C is driven to ground. This causes a conditioned voltage varying with ½ VDD for the pair of integrating capacitances of to driven ontonode 1413B much like the integratingcapacitance FIG. 3 . Thus, measurements of voltage taken at I/O 1404B will be with reference to ½ VDD. Since the threshold (e.g. CMOS) voltage of I/O 1404B is also proportional to ½ VDD in some implementations, changes in the threshold voltage can be compensated for by the reference voltage such that measurements of charge on the integrating capacitors are consistent. - Turning now to
FIG. 15 , another embodiment of a capacitive sensor 1500 is illustrated. In capacitive sensor 1500 amultiplexer 1514 is shared, and allows an integratingcapacitor 1520 and charge changingcircuit comprising capacitor 1526 and I/O 1504 to be shared by multiplemeasurable capacitances 1501A-D for sigma delta measurement. Themultiplexer 1514 is controlled by I/Os measurable capacitances 1501A-D. Typically, each of themeasurable capacitances 1501A-D would be independently measured in sequence, although they might also be measured differentially or a coded sequence could also be used to make independent measurements. Themeasurable capacitances 1501A-D not being measured can be coupled to a supply voltage while the other is measured.Measurable capacitances 1501A-D might also be sampled in some changing order or differentmeasurable capacitances 1501A-D might be sampled at different times within a filter window to perform analog filtering by sum, or difference. Note that some multiplexers may require more control circuitry (not shown), and that a variety of states and connections (such as all shorted, all open, and others) may also be used. In this embodiment, I/O 1516 is used in a low impedance state during sharing, and can also be used for measuring the voltage on the integratingcapacitor 1520. - Another variation of the embodiments of the invention is the use of dithering to reduce susceptibility to noise. Some sigma-delta based capacitive sensors can be susceptible to repeating noise spikes, commonly referred to as “tones”, caused when low frequency signals are applied to controller inputs. These tones can result from the repeating patterns caused by voltage referenced modulation. In some cases these tones can extend beyond noise floor of the sigma-delta modulator that includes the controller I/Os, potentially leading to output errors.
- One way to reduce the effects of tones is to add dither noise to the reference voltage of susceptible comparator outputs. For example, a single noise source can be applied to and used to reduce tones in multiple comparators. Turning now to
FIG. 16 , another embodiment of acapacitive sensor 1600 is illustrated. In this variation sensing system of 1600 is modified adding adithering circuit 1645. - The
dithering circuit 1645 includes dithering I/O 1630,capacitors impedance 1635. Thedithering circuit 1645 is used to reduce the effects of low frequency noise called “tones” on the filtered output. Specifically, thedithering circuit 1645 is used to provide a variable offset to the charge on the integratingcapacitor 1620. The variable offset changes the charge, effectively moving the reference voltage up and down. Thedithering circuit 1645 is preferably operated such that no net amount of charge is added to the integratingcapacitor 1620 over a measurement cycle for a selected measurable capacitance. This can be accomplished with a variety of different waveforms, including ramps, pseudo random noise, and the like. During operation, the voltage on the ditheringcircuit capacitance 1628 may be measured and changed by dithering I/O 1630. In one embodiment, the ditheringcircuit capacitance 1640 in this embodiment is larger than thedelta capacitance 1626, anddelta capacitance 1626 is comparable to ditheringcircuit capacitance 1628. By controllably providing voltage through the dithering I/O 1630 the desired voltage offset waveform is generated and applied to the integratingcapacitor 1620 throughcapacitor 1628. Also, in this embodiment, I/O 1616 is used in a low impedance state during sharing, and can also be for measuring the voltage on the integratingcapacitor 1620. - In other embodiments of a
dithering circuit 1645 the reference voltage of the integrating capacitance 1620 (e.g., ground, or the output of an I/O 1616 coupled to the integrating capacitance 1620) can be changed during a digital filter window for the measurement of ameasurable capacitance 1601A-D. In yet other embodiments the threshold voltage of the I/O (e.g. 1608, 1606) could vary over the digital filter window. Each of these methods and others (including those using DACs, current sources, and other active circuits) allow for “dithering” of the offset from the integrating capacitor to reduce the peak noise in the circuit due to “tones” in the filtered output of the sigma delta capacitance modulator. - Turning now to
FIG. 17 , another embodiment of acapacitive sensor 1700 is illustrated. InFIG. 17 a switched capacitor implementation of a sigma delta measurement system sharing an I/O within the charge changing circuitry. Specifically, thecapacitive sensor 1700 shares the I/O 1714 between two sensing channels two sensing channels (represented bymeasurable capacitances 1702A-B). The shared I/O 1714 is used to apply voltages to thedelta capacitances 1726A-B. In general, this embodiment changes phases to determine whether or not a particular integrating capacitance (e.g., 1708A or 1708B) in the passive network (e.g., 1709A or 1709B) is sensitive to a transition on the corresponding delta capacitance (1726A and 1726B). Specifically, each integrating capacitance can selectively share charge or block charge transfer from the measurable capacitance or the delta capacitance depending upon which side of the integrating capacitance is driven at a low impedance. Thus, the voltage on each delta capacitance (e.g. 1726A-B) can be allowed to transition without affecting the accumulated charge on the respective integrating capacitance (e.g. 1708A-B), and the I/O 1714 can be shared with multiple sensors reducing pin count. - Turning now to
FIG. 18 , another embodiment of a capacitive sensor is illustrated. Specifically,FIG. 18A illustrates acapacitive sensor 1800 that utilizes sigma-delta measurement techniques implemented in a controller to measure capacitance, and shares components to reduce device complexity and improve performance. Specifically, thecapacitive sensor 1800 includes six measurable capacitances (CX11, CX12, CX13, CX21, CX22, CX23) configured as transcapacitances between three shared transmitting I/O's (I/O-T1, I/O-T2, I/O-T3) and two shared receiving I/O's (I/O-X1, I/O-X2) and their respective electrodes. Thecapacitive sensor 1800 uses two shared integrating capacitors (CI1, CI2) and two shared delta capacitors (CD1, CD2). Thecapacitive sensor 1800 uses two sharedbiasing circuits circuits nodes capacitive sensor 1800 shares a delta I/O (I/O-D) among all six measurable capacitances. Thus, thecapacitance sensor 1800 shares a variety of I/O's and capacitors among six channels of measurable capacitances. - As stated above, the six measurable capacitances (CX11, CX12, CX13, CX21, CX22, CX23) are configured as transcapacitances between three shared transmitting I/O's (I/O-T1, I/O-T2, I/O-T3) and two shared receiving I/O's (I/O-X1, I/O-X2).
FIG. 18B illustrates aphysical representation 1870 of how the measurable capacitances would be implemented between the transmitting I/Os and the receiving I/Os. Note that the number of measurable transcapacitances is larger than the combined number of transmitting I/Os and the receiving I/Os. Thus, this technique can reduce the number of I/O's required to measure a given number of capacitances. This advantage grows as larger numbers of transmitting I/Os and receiving I/Os are used. However, this technique can require additional measurement cycles to measure all of the measurable capacitances. - During operation of
capacitive sensor 1800, a voltage is repeatedly applied to a selected measurable capacitance using a corresponding I/O, and charge is repeatedly shared between the selected measurable capacitance and its corresponding integrating capacitance to accumulate charge on the integrating capacitance. Furthermore, the charge on the corresponding integrating capacitance is repeatedly changed by a quantized amount of charge using its corresponding delta capacitance, with this charge changing responsive to the charge on the integrating capacitance being past a threshold level. The results of the charge threshold detection are a series of quantized measurements of the charge on the integrating capacitance, which can be filtered to yield a measure of the selected measurable capacitance. -
FIG. 18C includes a state diagram 1875 that illustrates a exemplary state sequence forcapacitive sensor 1800. Referring toFIGS. 18A, 18B , and 18C, thestate sequence 1875 provides measurements of capacitances CX11 and CX21 using transmitting I/O T1 and receiving I/Os X1 and X2. Typically, thestate sequence 1875 would be repeated multiple times for an accurate measurement of capacitances CX11 and CX21, and then a similar procedure would be repeated using transmitting I/O T2 to measure capacitances CX12 and CX22, and then similarly repeated using transmitting I/O T3 to measure capacitances CX13 and CX23. In other embodiments the capacitances coupled to multiple transmitting I/Os could be measured concurrently using differential measurements or coded modulation/demodulation of the transmitters. - The
first state 1 comprises an intermediate high impedance state. In this state, the biasing I/Os S1H, S1L, S2H, and S2L, and the receiving I/Os X1 and X2 are all held in a high impedance state, with the delta I/O driven high and the transmitting I/Os T1, T2, and T3 driven low. This results in an intermediate state that decouples the various capacitors to temporarily trap charge in those capacitors, and assures that there are no overlapping signals that could otherwise inadvertently set an unwanted charge on a capacitor. - In the
second state 2, the voltage on the integrating capacitors CI1 and CI2 are set to a generated bias voltage VG implemented to substantially equal the threshold voltage VTH of the measuring I/Os. Specifically, I/Os S1H and S2H provide logic high voltages (e.g. VDD), I/OS S1L and S2L provide logic low voltages (e.g. GND), and resistances in biasingcircuits nodes - It should also be noted that ½ VDD is just one example generated voltage, and in other embodiments it may be desirable to use other values. For example, in the case where the I/O's utilize a Schmidt trigger input a voltage of ⅓ VDD might approximate the input threshold of an I/O which was just set to a logic high.
- As will be explained in greater detail below, driving
nodes states nodes steps 7, 8 and 9). Keeping the voltages at 1898 and 1899 constant makes parasitic capacitance to fixed voltages (e.g. GND) largely irrelevant since charge moving through the parasitic capacitance is minimized. - In the
third state 3, charge is shared through the measurable capacitances CX11 and CX21 and their respective integrating capacitances CI1 and CI2. This is accomplished as the transmitting I/O T1 transitions from a logic low voltage to a logic high voltage. This adds charge to the integrating capacitances CI1 and CI2 through CX11 and CX21 whilenodes - In the fourth and fifth states, a controlled amount of delta charge is similarly transferred to the integrating capacitances CI1 and CI2 through delta capacitances CD1 and CD2. The amount of charge transferred depends on previous measurements (outputs of the quantizer) of the voltage on the integrating capacitances measured at
nodes capacitances nodes nodes state 5, charge is changed (e.g. removed, or not removed) from the integrating capacitance depending on the previously set states of the associated biasing I/Os (which are in turn dependent on the previous quantizations of charge on the associated integrating capacitances). In order to use the same delta I/O, the sharing of charge through the delta capacitors is allowed or not allowed by driving thenodes - As one example, the functions are selected such that if the voltage quantized on one or more of the integrating capacitances (e.g., at
node 1898 or 1899) was higher than the threshold voltage VTH for the previous repetition of the measurement cycle (i.e., the charge on the integrating capacitance is low), then based on that output the biasing I/Os float theirrespective nodes node 1898 or 1899) was lower than the threshold voltage (i.e., the charge on the integrating capacitance is high), the biasing I/Os provide a low impedance to theirrespective node
F 1H(V CI1)=V DD and F 1L(V CI1)=0 when VCI1<VTH
F 1H(V CI1)=Z and F 1L(V CI1)=Z when VCI1>VTH
F 2H(V CI2)=V DD and F 2L(V CI2)=0 when VCI2<VTH
F 2H(V CI2)=Z and F 2L(V CI2)=Z when VCI2>VTH Equation 1 - Thus, states 4 and 5 selectively remove charge from integrating capacitances CI1 and CI2 using their respective delta capacitances CD1 and CD2 based on functions of the previous voltage measurement(s) (e.g. quantized outputs) at the integrating capacitances.
- The
sixth state 6 comprises another intermediate high impedance state that assures that there are no overlapping signals that could otherwise inadvertently set an unwanted charge on a capacitor. Theseventh state 7 sets the receiving I/O's X1 and X2 to a logic high voltage to prevent charge sharing to the integrating capacitors. The eighth state 8 sets the charges on the measurable capacitances and the delta capacitances (CD1 and CD2) in preparation for transitions on a following repetition of the charge transfer process. Specifically, a logic high voltage is put on I/O D while a logic high voltage is also put on I/Os X1 and X2, discharging delta capacitances CD1 and CD2. At the same time a logic low voltage is placed on transmitting I/O T1, recharging the measurable capacitance. By putting a low impedance voltage on receiving I/Os X1 and X2, and high impedance on the biasing I/OS S1H, S1L, S2H, and S2L, charge will not be transferred onto the integrating capacitances during this step through either the delta capacitance or the measurable capacitances. This assures that the value of the integrating capacitance remains an accurate representation of the transferred (shared) charge accumulated during previous steps, and that it can be measured without being disturbed by noise from sensing electrode. - The next state 9 measures the voltage at the integrating capacitances VCI1 and VCI2. With the biasing I/Os at a high impedance state, the voltage (due to the integrated charge) on the integrating capacitances (e.g., the voltage at
node 1898 and 1899) can be measured at any of their respective biasing I/Os. This quantized measurement can comprise a comparison of the voltage at the integrating capacitance with the threshold voltage VTH of the biasing I/O to provide a quantized output (or result). The resulting quantized measurement of the voltage on integrating capacitances (i.e. whether they are higher than threshold voltage VTH) may then be used in functions F(VCI1) and F(VC12) in the next cycle to determine how the charge on integrating capacitance might be changed by the associated delta capacitances. - Thus, the repeated execution of states 1-9 will result in sigma-delta closed loop control of charge on the integrating capacitance, and a filtered measurement of the quantized results can be used to measure the transcapacitances CX11 and CX21. Then, the process can be repeated multiple times for an accurate measurement of capacitances CX12 and CX22 using transmitting I/O T2 as transmitting I/
O T 1 was used in the illustrated states 1-9. Then the processes can be similarly repeated using transmitting I/O T3 to measure capacitances CX13 and CX23. The resulting measured transcapacitances can further be used to sense the proximity of an object relative to the sensor. - The use of the
capacitive sensor 1800 to measure transcapacitance rather than absolute or ground referenced capacitance can provide significant advantages. Specifically, it can reduce the negative effects of background or parasitic capacitance on the measured capacitance (even without using a controlled guard waveform) and thus are particularly usefully in applications where there is a higher proportion of parasitic trace capacitance, such as fingerprint ridge sensing and capacitive touch sensing. - For example, when driving a generated voltage VG on
nodes - Other variations of
capacitive sensor 1800 embodiment can be implemented. For example, in some embodiments the transmitting I/Os can be implemented to transmit with different voltages concurrently to facilitate differential measurements. In other embodiments the controller can be configured to selectively sample and integrate on the integrating capacitors, thus not sampling and integrating on every cycle. Furthermore, in the illustrated embodiment, charge is only moved through each delta capacitor CD1 and CD2 in one direction, but in other embodiments it could be implemented to apply an opposing charge through the delta capacitors. - Thus, the embodiments of the invention provide methods, systems and devices for detecting a measurable capacitance using charge transfer techniques that can be implemented with many standard microcontrollers, and can share components to reduce device complexity and improve performance.
- Various other modifications and enhancements may be performed on the structures and techniques set forth herein without departing from their basic teachings. Accordingly, there are provided numerous systems, devices and processes for detecting and/or quantifying a measurable capacitance. While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. The various steps of the techniques described herein, for example, may be practiced in any temporal order, and are not limited to the order presented and/or claimed herein. It should also be appreciated that the exemplary embodiments described herein are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Various changes can therefore be made in the function and arrangement of elements without departing from the scope of the invention as set forth in the appended claims and the legal equivalents thereof.
Claims (19)
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